I hear ya man. So your car is turning into a race car having a trailer for it. Unfortunately I am running with an 8-1/4 and having a shop install it. This is why I am making a big deal about it.
Might want to stick with the 3.55s for a while if you have them. If that engine is making any decent power and you get it to hook, that 8 1/4" diff may not like it and you'll be in the market for an 8 3/4". You should be able to get an idea on your converter stall by standing on the brakes in drive and loading the engine, it will stop revving somewhere around your stall speed and give you an idea if it's 2500, 3500, or whatever. Don't do that more than a couple seconds or the trans may not like it, and check your driver side motor mount afterwards.
FWIW...I have a 340/727 with 3.55s on a 29" tire and I don't think that I would be comfortable at 75mph+ for more than half an hour on the highway with it. It's right smack in the power band, but it's starting to get loud and such. On the back roads, no issues and I could go 3.91 if I know I'm not going cross country on the highway.
