Got Shell 93 Octane and now barely idles 1973 Dart 340

So now you are up to 17 inches at 850 and 12 degrees initial.
I'd call that a stock cam.Or real close to it.
In which case, I would set the power-timing to 34/36, and let the initial fall where it may.
If you mark your balancer, with the hillbilly timing tape method, be advised that, after 3 copies, the 40* mark can be off by several degrees. The marks will be exactly as accurate as you make them.Stock 340s like a lot of power-timing.If your 36* mark is closer to 38 or 40, well, don't be surprised if things get expensive.
Timing tapes are like 7 bucks.....