Welcome to the forums. Based on your posting it sounds like the engine was already yanked when acquired it . . . do you have the original crank pulley? In this era both Chevy and GMC used a combination harmonic-balance-pulley. So if you have the original I would stick with it. To Beaters point one can still get 5/8 belts and retro the 5/8 generator pulley onto an alternator.
Are the exhaust and intake manifolds missing entirely?
