IMO A stock 350 is in most cases not going to take 20 degree base timing. Worth a try but it could be a bear to startup when hot. Tend to ping itself silly up hill in the wrong gear etc. But it all depends on the rest of the tune and other factors.
14 ish initial/base should be plenty as a performance tune stock engine. Much is dependant on vac advance ported or manifold because if manifold then the idle timing is base + vacuum adv. The engine vac should be pulling 17 "wg +. The vac advance will be all in at idle no matter what. So keep this in mind when factoring in everything.
Run it on ported and at light trottle you are still way high with mechanical coming on,,,it could rattle like a nursery. The cam is tight and cylinder pressure maxing out at low rpm. etc.
Too much emphasis on base or initial seems to be the going concern. Target the total timing first (initial + mech) is 35 the magic number at 3500 rpm and beyond? do some WOT pulls. When you know this number 32-35-38 whatever keep it as gospel, everything else on the distributor revolves around it. (no pun
You have 25 deg of mechanical, so how much vacuum adv? could be another 20? test it out, maybe you have no vac adv? tell us.
This is 1974 car so just shy of HEI, what type of dist?
I am unwise to the ways of corn gas, but have had a laddle or two of corn squeezings on occasion