Got Shell 93 Octane and now barely idles 1973 Dart 340

OBTW, I made a mistake: turn the crank CCW to make the 10-20-30 BTDC marks, not CW. Sorry. I'll correct the prior post. I woke up in the wee hours realizing I'd told you wrong; it was 3:40 AM oddly enough LOL

Keep in mind that total timing has to be kept in mind. If you have a stock distributor with 20-22 degrees mechanical advance, then if you ended up with 20 degrees initial timing then the total would be 40-42. OK for light cruising, or a low compression truck engine (maybe!), but well past the recommended maximum of the mid 30's for an engine that gets beat upon and with some reasonably high compression. So, since you don't know your distributors mechanical advance curve, then the low 10's for initial timing is where I would stay. We are at 15 initial with a 10:1 CR 340, but we also have AL heads and quench, which help to combat detonation. 'It' (the engines idle) would like more, but we knew we were pushing the limits of advisable total timing, without modifying the mechanical advance. Your engine has unknown internals; just IMHO, better safe than sorry.