Vacuum/Mechanical Advance Dilemma SB340

17 and 31 are both "somewhat" low. I would try more initial timing and that of course will raise total. Small blocks generally like total timing figures in the 34-36 degree neighborhood. If you can run a compression test, that would give us more information to work with. It's probably a low compression build from your description so far and there's nothing wrong with that. They can be made to run well. They just need a lot of initial timing. If you can also tell us "what cam" and "WHERE" it was installed. In other words was it degreed or just slapped in dot to dot? Remember too, that vacuum advance is never included when timing an engine. It's initial and mechanical only and vacuum advance "just is what it is".