If…if this distributor is set up correctly then manifold vacuum is best. During acceleration the mechanical advance takes over as vacuum drops with open throttle. More expense but If this engine has poor vacuum at idle due to performance cam then a programmable distributor that allows for a simulated advance at idle then a preset all in advance programmed at WOT. There are adjustable vacuum advance cans. LT-1 350 (1970) had one of the best factory distributors with a correct vacuum can and mechanical advance. (Nostalgia) it was manifold vacuum.I don’t disagree with you on that, but this is clearly not the stock engine for this car. My experience with SBCs (especially ones with aftermarket cams) is that they do well with ported vacuum advance. Your mileage may vary - nothing wrong with trying it both ways (will need to adjust timing and carb accordingly, and see what drives best.