What would the best initial timing be on my engine?
340 running an edelbrock performer RPM cam. MP single plane air gap intake, stock heads, 10.5:1 pistons. I am thinking somewhere in the 15-20 before?

How much mech adv do you have in the dist?

I am not sure it's a stock 1970 points disty.

With vac adv plugged, use a dial timing light, set the total adv (int & mech) to 35* @ 3500 rpm, if it pings under load, start backing it down till it stops, you can then check to see what int adv you have by checking it at idle. With 10.5 comp, you may have to run 30* total, maybe even less, higher octane fuel will allow more timing. If you want more int adv, you'll have to reduce the amount of mech adv. Normally, the more int adv you have without detonation, the better.