Early on in the emissions game, it was found that if you delay the spark, some of the fuel would then burn in the exhaust system and reduce the bad things Big Brother was looking for. This was at a loss of fuel mileage and power mind you. It also caused the engines to run hotter. If you have problems running manifold vacuum, you may have an "Emissions" distributor. In the case of the 65' Buick, Was it a California car? 1971 was the first year for an across the board emission's reduction for all 50 states. No dought, the factories where working on the problems, trying to prevent Big Brother from stepping in quite a bit earlier in the game. California was always first in line for that sort of thing anyways. I know the smog pumps and such showed up several years earlier in California than the rest of the states.
Most "Smog" distributors that I have seen don't have much mechanical advance at all. (A distributor with worn, rusted, or seized weight would act this way as well). A high number would be 20* at the crank. Then they have real stiff springs that need a whole bunch of RPM before you get full advance. If you have the initial set at 8*, you may only see 18* @ 2500 RPM, 23* @ 3500, and 28* @ 4500. Not near enough for any combo I can think of. In this case the ported vacuum would help your performance by bringing in more timing. Thing is, you crutched the problem and didn't fix it.
You will also see guys and gals who advance the initial timing to the point that it is hard to start. And the motor still wants more. Lighter springs are then added to allow the mechanical advance to come in quicker. You then run into the stumble, surge, and miss problem from having way to much timing advance. The motor runs much better at low RPMs with the added timing, but at higher RPMs, the timing is still being piled on by the vacuum can.
You read posts of timing woes quite often. You also read of those who feel Ported vacuum is the best. In my eyes, manifold vacuum is the only way to go. More power, better fuel mileage, and cooler running..........Dan