And to keep adding...........typical tube size for the two side tubes is 3" the length of each. However, there are some '70s vintage housings, mostly from Torinos, that have a neck down toward the brake end (this is the tube, not the bearing Tom mentioned). It's not much of a problem, but could make for an odd deal depending on shortening process. Speaking of which, a qualified guy will have a fixture which consists mostly of a large, straight bar and some bearing sized end adapters to hold the housing straight when rewelding the ends on after cutting. I've known one guy who had the experience to heat the right places to re-tweak after welding to straighten a housing, but that kind of experience is not very common.

There is no "perfect" width, though any of the three you mentioned will work well enough. There are lots of choices of e.g.; fendered of hiboy? Size of tire, and even more important wheel offset; mounting method of rear end and it's components. I would probably go for the 56" myself, but I like a deep set face on a wheel, and understand how to account for wheel back space and tire bulge.