Cool thought, but it doesn’t seem to be working. How about something like this, it would just require some hose work. If it solves the problem you could work out a solution you liked better.

https://www.jegs.com/i/JEGS/555/53098/10002/-1?CAWELAID=1710676818&CAGPSPN=pla&CAAGID=39714413703&CATCI=pla-457013475083&CATARGETID=230006180040357466&cadevice=t&jegspromo=nonbrand&gclid=EAIaIQobChMIxd-B7d7K3AIVBcpkCh2BCwtQEAQYESABEgJGlPD_BwE

"But I also must say my filling point is lower than the radiator. The system my dad and I designed is different. It has a separate filling reservoir. I don't fill up the top tank on the radiator. So I'm thinking I'm getting a little steam bubble up too and it pushes out the coolant when I turn up the RPMS. So I think I'm planning on putting some kind of bleeder or plug on the top tank to be able to let air out."

Do you have a coolant overflow/recovery tank in the system in addition to the expansion tank shown?

It would be a shame to have to remove that expansion tank (the inline filler mentioned is the quickest out without having to modify the grille shell).

My unasked for opinion is the simplest all around best looking system is the one the car came with. Any other system only adds more unnecessary clutter and results in a system that doesn't work as well as prior to the modification. I have a mostly original (sheet metal wise) Y-block powered 32 5w, see the radiator and all every day. I see nothing objectionable about the original form. Still using an original radiator by the way.  

The glass T-bucket I had was a Total Performance unit from before the sale to Speedway. Apparently they had the same idea of cleaning up the top of the shell also. Didn't work there either. The way I fixed it, with the cap I used, the top of cap was barely above the top of the shell. Actually was cleaner looking than before because there was no filler neck sticking out the back of the expansion tank. And I was able to have probably about 1 1/2 quarts more coolant in the system. 

The original type system was better on every count in my book. 

My 32 has a “filler neck” like your talking about adding, but it’s got enough radiator to manage the expansion with a small overflow/return bottle. It’s mounted low, like most vehicles. While I agree the system isn’t set up to allow proper fill/expansion, it might also be your lacking volume or cooling capacity. I understand your not afraid of fabricating something, but I’d stick on the filler neck up high and just connect it with hose. Then I’d live with the ugly until I knew the problem was solved. If it isn’t, there’s more work to do. It will also let you jury rig a catch can in and get some idea of the expansion volume you need to keep the system full.

I don’t know if this is helpful, but years ago I had a fairly new Volvo that used a pressurized overflow tank. The radiator had a sealed cap, just like earlier models had a pressure cap. The overflow tube connected to a tank mounted alongside the radiator and it had the pressure cap. Kind of clear plastic, it was marked minimum and maximum. The only time you unsealed the radiator was to drain to flush it, otherwise it was done thru the tank cap. I wasn’t real hot about a plastic tank being pressurized, but like most things Volvo in the 60’s it just worked. It took a while to bleed the air out if you started from empty, but part of that was the heater was higher than the radiator.

Once you know you do have cooling capacity, you can fab a pretty solution, maybe still using your expansion tank.

This picture is of a couple year newer Volvo, and has a pressure release it looks like on the radiator. Mine didn’t, just a sealed cap.
But it was what I could steal off the net.

Of course you can absolutely do whatever you wish. 

But it seems to me whatever it all is you are going to add is not going to change anything except to increase complexity. The coolant level will be the same or higher than it was. If the little tank and pressure cap remain the same as they were then the pressure in the system will still cause a leak there same as before. 

Makes no sense to add parts to bleed off pressure. It's there for a reason. I live in central Florida. 2 Y-blocks, both run 7lb. caps. Never any problems on either of them. Not at all sure you need the 16lb. cap you are running. If you want less pressure try a lower pressure cap.

But the cap simply needs to be at the highest point of the system with enough space somewhere to account for expansion of the coolant. As I said at first, that system is designed to leak . That's why it hasn't been fixed with everything you have tried so far. The actual problem has not gotten any attention.

No real point trying to re-invent something that's been figured out for a very long time. The cooling systems for engines started out real simple and they worked well. Especially on hot rods. Expansion tank on top, low or no pressure cap. As time went on they became more and more complex for all kinds of reasons. Not many (there are a few) of which have anything to do with hot rods. But if that's what you're after go for it.

Early 60s Y-block Fords used expansion tanks. And the FE's too. Find one or find good pictures of some and copy what they did if you can with the placement of your parts. Don't think you be able to. Or want to. And who needs it anyway.

That's all I got.

My 32 has a “filler neck” like your talking about adding, but it’s got enough radiator to manage the expansion with a small overflow/return bottle. It’s mounted low, like most vehicles. While I agree the system isn’t set up to allow proper fill/expansion, it might also be your lacking volume or cooling capacity. I understand your not afraid of fabricating something, but I’d stick on the filler neck up high and just connect it with hose. Then I’d live with the ugly until I knew the problem was solved. If it isn’t, there’s more work to do. It will also let you jury rig a catch can in and get some idea of the expansion volume you need to keep the system full.

I don’t know if this is helpful, but years ago I had a fairly new Volvo that used a pressurized overflow tank. The radiator had a sealed cap, just like earlier models had a pressure cap. The overflow tube connected to a tank mounted alongside the radiator and it had the pressure cap. Kind of clear plastic, it was marked minimum and maximum. The only time you unsealed the radiator was to drain to flush it, otherwise it was done thru the tank cap. I wasn’t real hot about a plastic tank being pressurized, but like most things Volvo in the 60’s it just worked. It took a while to bleed the air out if you started from empty, but part of that was the heater was higher than the radiator.

Once you know you do have cooling capacity, you can fab a pretty solution, maybe still using your expansion tank.

This picture is of a couple year newer Volvo, and has a pressure release it looks like on the radiator. Mine didn’t, just a sealed cap.
But it was what I could steal off the net.

I guess what I’d do is put in the inline cap and eliminate the tank for now. It’s the easiest way to find out I’d the cooling system works. You can run a hose to the catch can, verify it’s got enough volume, and make sure the vehicle cools. That way you've got a baseline that “works” for minimal cost and effort. Then you can do what you like, and if it doesn’t “work” take another look at the design.

Like Roger points out, simple is probably the best.

My 32 is setup as Miker mentioned with the fill on the top tank offset. It works great.
I also have an overflow tank that works well. I run a 12 # cap