55.2-59 changing drivetrain for 1958 Apache 38

[dsraven]

was the motorhome a class A or C? width may be too wide to look right under the old truck.
the reason I ask about the brake size is that the larger motorhome axle, if it was a class A, may have large wheel cylinders to operate the larger brakes. your master cylinder may be too small to operate them properly and/or the brake bias with the larger rear brakes may not be a good thing.
anyway, that could be down the road a way. I suggest, if you are gonna do the engine swap, is get that done and operating well, all the bugs ironed out, then worry about the rear axle or the gear ratio. in the mean time get the width of the stock axle from 58 and keep that in mind when out doing errands or whatever. look for a modern truck with dual wheels that may match up to the width you need. when you see one that looks right google the track width and see how it matches up.
I guess it boils down to the question of what do you plan to do with the truck when done to your satisfaction. if you want to just use it locally around town and in the area, not for longer highway trips, then the v8 will get you the power you want and an overdrive trans or bolt in overdrive will help with the highway speed. after that, if high speed is what you want, you really should think about uograding a few things to handle the steering and braking. if you wanna go fast you also need to be able to stop fast and have a truck that will be safe at the higher speeds