Welcome aboard! You're not going to find an alternator specific to the application, only part available is the brakcet to adapt one to the engine. The brackets are usually listed wiht what alternators fit them, but you'll most likely be looking for a 10Si or 12Si from a late '70s to early '90s GM, as your rebuild guy mentioned. Pretty much any of the aftermarket hot rod suppliers, like Summit, Jegs, and Speedway, carry them, just a matter of what amperage output you want. There are also conversion kits available to make your own froma stock replacement.
Now, if you want to keep a the generator look, but have an alternator, there is the PowerGen alternator. It's basically a GM style alternator inside a reproductin generator case, however, they are very expensive.
Personal opinion time; I tried a single wire alternator on mine. It did work ok, once I got the pulley size sorted out (do NOT use the stock generator pulley, too large), it didn't seem to respond quick enough from idle. converting back to the stock style 3 wire regulator corrected that problem. I don't mean to put single wire alternaors down, or say not to use one, only relating my own experience with them.
