1968 340 timing
IMO the only time a streeter would run manifold vacuum advance is with a cam that is waaaay more strip than street.
When yo do that with a typical street carb, it is impossible to correctly set the transfer slot exposure, without also simultaneously having a way too fast idle speed.
Your engine does not care one bit about Power-Timing, until it hits stall. Punching it up below stall is nothing but trouble...
With a manual trans and a moderates cam, it will just exacerbate bucking.
But hey, you've thought about it so your way must be right. And it'll only cost you time,to start over.
I think it was
@toolmanmike who said more is not always better, and
when it comes to timing,
that is 100% true.