INHO, forget the initial timing. Get a dial back timing light or a timing tape. SBC, unless it is a newer vortec engine, like 34-36* total timing @ <2800-3000 rpm. Total timing is centrifugal + initial without vacuum advance hooked up.
If you set the timing this way, forget about what the initial timing is. Just make sure you do not exceed 34-36* at higher rpm. This number is for maximum timing regardless of maximum rpm. It may take a distributor kit to accomplish this. Also, this is a base setting. For maximum performance you will have to dial it in as each engine is different.
Vacuum advance is a whole different ball game. Either use a Crane? adjustable vacuum advance kit
or a correct GM vacuum advance unit based upon your idle vacuum and limited to 14*. This will give you 50*. Use manifold vacuum not ported.