May have already been said, but a big block mopar has the crank buried in between the block skirts. Makes it hard to get the oil off the crank so not so much to gain dry.

sure 20+ inches of vac will do more than anything else for power, but you can get away with 15 wet so I don't see it being worth it unless you need a lower crank center.

Sure vac helps the rings seal a bunch, but I think most of the power is from not having to fight the air like a plane propeller inside the crank case. Think how much power it takes to run a fan 500 rpm. Now imagine a crank coated, and slinging oil at 9,000 rpm. 21 inches is near outer space and near zero air drag.