670 vacuum secondary vs 650 double pumper

One of the problems with a vacuum secondary on a 4-speed, is that it takes a moment for the back barrels to open. So, every time you let off the gas to shift, your back barrels are going shut and then you are waiting for them to re-open again. Because it's a 4-speed, you would want the dp. If you already have the 650 dp, go with it. If you haven't bought one yet, I'd go with the 750 dp.