Lots of questions; I think I can only answer one of 'em, though. I'm pretty sure that the GMC and Chevy sixes all use the same bellhousing bolt pattern, and SBC is different. So you'll need an SBC bellhousing. But the 4-speed tranny will bolt up; they're all the same. You may want to consider one of the pressed-steel bellhousings from Summit or any speedshop, they can be drilled and welded as needed for "minor" adaptations.

The 58 was available with a V8, so you should be able to find motor mounts. As for the rear axle, heck, just let that ol' SBC spin up some! With good rods and pistons you can cruise at 4500 RPM.... ;-)

George