Where to set timing

Forget the factory specs, those were far too sedate. The basic answer is "as much advance as you can without pinging problems". That is how modern engine controllers work. They keep advancing the timing until the knock sensor tells it to back off, then they creep up again. Read the Holley Commander 950 manual (online) for more info. Of course, you don't have feedback so must allow headroom for worst-case (hot day, WOT). Too much initial timing can make it hard to start. Ideally, you would want to spark slightly ATDC to aid the starter while cranking.

There was a plan for a simple box to control spark, termed Rabid Gator, via a GM 8-pin module. But, they disappeared. Someone will eventually make such. MegaJolt is the only "spark only" controller I know of, but pricey.