You were asking about vac advance, it's purpose is economy. That is it's main advantage.
In more radical applications, it does very little, while on a more stock application there is no real reason NOT to run it, as it will help with economy, and keeping the plugs clean..

I think to make a better timing recomendation, we need some cam specs, and gears, converter stall speed etc..



If the motor is mostly stock (which it sounds like with a low rise intake, iron heads, and a 750 eddy carb) with matching stock/mildish stuff, like say a 268 advertised duration cam, a stock stall and 323 gears, then 10 to 14 inital timing, with a total of 34 or 35 all in by 3000 rpm or so. Add on maybe up to 10 or 12 in vac advance to help it out at cruise.

Same motor with an mp 509 cam, 4000 stall, and 411's will need more like 22 inital, 35 total all in 2500, and no vacuum advance needed.(and will also work better with a bigger carb and manifold )

The main thing, to remember, is to shorten the advance slots, so you don't have too much total with the base timing you use.

Total timing is often overlooked, and is one of the most important numbers