Curious what I can expect from my std. 3 speed gearbox in my 55'Tbird?

I think your transmission is the T86. Ford went to the T85 which is a good solid unit. If you could get a T85 with Overdrive you would have a real nice set up.  

The casting numbers are 51R and its a side loader.

Back in the 60's, I put a T-85 R-11 in my 55 bird. It isn't an easy or pleasant experience, but it fit. I think they made some mods in 56 to allow it to fit better. They were used thru the 406 FE's, unless you paid for the T-10 option. As to the T-86, it depends on how hard you drop the clutch. They're a good trans, just not up to a lot of abuse. I ran one behind my supercharged (Paxton) 292 stock motor for a couple years with no problems. None of these have a synchro 1st gear.

Given what I think you're building, you might want to bite the bullet for a WC T-5, or a TKO500. Look close at the T-5 models, some were made for 4 bangers, and they won't live. The T-5 in my 'bird was an easy fit with Mummert's adaptor (watch the imput shaft length), but the TKO500 in my '32 was tight. I wouldn't do that again, and I'm not sure how it would clear in the bird. Different ratios for gears, wide and close ratios. The T-10 4 speed is also an option. Some of the later trans require turning down the input bearing holder to seat in the bell housing, and maybe drill and tap a couple bolt holes. Not complicated.

In the final analysis, what breaks first? The Dana 44 rear is pretty stout, so, u joints will be next if the trans holds up.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

If you have the original transmission the simple answer is no it will not hold up. We used to call them a butter box for a reason. The stock 292 2 barrel in my '58 Ford Custom would destroy them on the 1-2 shift. 2nd gear and the cluster gear would self destruct on a hard shift. The '56 and up T85 is good as stated. The all syncro 3 speed from a '63 and up big block is better yet.  A world class 5 speed is the cats meow if you do not mind having a shifter that does not look original. A big block toploader 4 speed can be installed and if you are handy you can weld  the stock bird shift handle to a modified Hurst shifter and have a bullet proof 4 speed that looks factory. Hope this helps.