Vacuum advance benefits in one state, cruise/constant state rpm driving. It's a mileage thing. Lean mixtures are hard to fire, so you have to give it a bunch more lead, enter the vac adv distributor. If you want to run the cruise at 15:1 or higher, by all means, get a vacuum advance distributor. Be VERY careful because that lean mixture, 12:1 compression, big cam, lots of advance may not play nice. It can be a headache getting it right and I've had cars that refused to accept any vacuum advance. As much as getting economy is a goal, I will not waste my time on certain deals that are potentially tempermental. It's a race engine, not a mileage master.
It shouldn't add any advance when starting because the can will require in excess of about 12-14" of vacuum to activate, unless it has a very generous adjustment in it. Where you hook it up, manifold or ported throws another wrench in that discussion.
You have a high winder and want to put a bunch of stuff on it that makes it a runner trying to run at the olympics breathing with a soda drink straw.
Getting to the Carb, I did 750 DP carb for a friend, 360 350hp/400tq, 3.00 gears IIRC, 904 auto with a 3K ish converter. Car got 20mpg on the freeway and ran around 108 in the 1/4.