I'd rebuild it - in fact I've done so, more than once, they're the best V8 GM ever made - the 58 has a rebuilt 283, 060 over, runs like a stuck pig and if they're sound, they don't mind the revs at all, with the stock 3.90 rear I've cruised at 70 for extended periods [3000rpm?] .... bit noisey, but won't hurt the engine
you're right that no V means it was a 6 from the factory, but of course anyone could have chosen the V8 after the fact, being the optional engine - could have even happened at the dealer when the original owner wanted that truck 'right now' but with a V8
the OD trans is a good option for lower revs on the open road, and might be easier to find - certainly easier to install than new gears and would drop the rpm from 3000 to about 2200, the 4 speed is only useful if you plan on crawling around the farm fields in the mud, otherwise it's a 3 speed with an xtra useless gear
Bill
