With any of the yellow springs, those end carbs should come on VERY quick. I went one step up to the purples to take a bit of hit out of the car.I have a Comp XTQ296 cam.....264/270 @ .050 and about the same vacuum reading as you. Stomp the loud peddle.......1.45 60' and HOLD ON. Make sure those end carbs cycle freely... and another thing that might sound stupid......make sure the base gasket is on the right way otherwise the end carbs will NOT open. I've ran into that twice on a couple of customers cars.
As said, engine vacuum has nothing to do with the end carbs opening.