Here is a little help on the rear end. 55 Chevys with the 6 and a 3 speed came with a 3.55 or a 3.70 rear gear depending on the model. Wagons were mostly 3.70 or 4.11 and that often had a 3 speed with overdrive for easier highway cruising. The cars could actually be ordered with anything from a 3.36 to a 4.11 and the center section was changed at the dealership when the car arrived.
Rear tires were 6.70 - 15 which is about the same size as on your 51 when new. I think it is pretty safe to say that in 55 a lot of owners drove their 235 and 3 speed jobs on the highways and you shouldn't worry too much about it unless you decide to run a lot of gear or the motor is getting tired.
I'd use a 55 to 57 rear end, preferably a 56 or 57 for the larger axle bearings. A rear from an automatic 8 cylinder would usually be a 3.36 and almost all standard shift cars 6 or 8 cyl had at least a 3.55.
