If cost effectiveness is the determining factor, then it is a question only you can answer.  Make your best effort to figure the total cost of the FE swap and compare that to your best effort to figure the cost of a Y that will perform acceptably to you.

When contemplating the FE, remember that it is not just an engine you will be buying.  You will need a whole lot more stuff to complete the swap. It will be mounts, transmission, driveshaft modification, clutch or automatic transmission linkage, throttle linkage, fuel, coolant, and possibly transmission plumbing, exhaust routing, at least a little wiring, etc.  And the list will not be limited to those.
When it is all figured, engine swaps are not usually the bargain they seem to be at first.

Regarding the possibility of having the original engine, the only way it stays original is if you don't touch it.  If you modify that engine, it is no longer original.  Another Y block will definitely swap in with no permanent modifications required to the car - you can always go back and have it be original.  With a swap to any other family of engines, there is always the risk that some permanent change will need to be made in order to get it installed and functioning correctly.  In my book, that means the car can never be considered to be original again.  Even if you go back and repair it, to look like new, it is no longer original.  So if originality is important too you, also make sure that the FE will go in without any alteration to the frame, body, or any other original parts and figure into the cost of the swap any parts you will need to make or have made to avoid altering original parts.

While you won't set speed records, 188hp should move 3600lbs nicely.  It should cruise at modern highway speeds, and should not be holding anyone up.  Since it does not seem to do that, it may not actually be the 188hp engine, it may be worn out, it may be improperly tuned, etc.

Worn out is unlikely, no outward signs of it (smoke, blow by, etc.). Poorly tuned is a possibility. Also the heads could stand to be freshened up. When I got it I found someone had set the valves at 0 lash. I backed 'em off to the proper lash but I may have one or 2 leaking.

What is your rpm at "engine-safe top speed?" 

3000 at 60 in direct drive. The speedo indicates 70 at that RPM but it reads 8 to 10 mph fast.

Unless you are geared real short, I don't see how you can be near any such rpm at only 50 to 55 in drive, 1:1.  Maybe it's not drive?

Dana 45 with 4.09:1 ratio. In OD it's effectively 3.73:1.

Expecting to cost effectively equal the torque of an engine with nearly 100 more ci is unrealistic, but I think a Y can work for you quite easily.

How much is the crate engine?

$3600 for a long block from CME in S.Carolina.

When I think a crate engine, I think of something like you might get from summit, that is not stock.

This one isn't stock but nearly so; has an RV cam and a few other minor tweaks. It's intended for a daily driver.

With a brief search, the cheapest I find is about $4000.  If I look for remanufactured engines, I find about $1600, plus intake, plus distributor, plus exhaust, plus carb, etc.  Add on top of that the cost to actually perform the swap, and that is a pretty sweet Y block.

The other possible fly in the ointment; finding a trustworthy local machine shop. I've talked to a few mechanics around here and none of them like any of the local shops (rural Middle TN).

I think for a fraction of the cost of the swap you can have a Y that will perform sufficiently, though probably not with equal torque.

I wouldn't expect torque equal to a 390. I'm just thinking a 390 might be the easy way to get there given the machine situation around here.