I was throwing my 2 cents in, I did the conversion too, I used the right stuff conversion kit and it came with a 9" booster for the same issue valve cover clearance. The kit had the same combo valve and it was causing my rear wheels to lock up if I put too much pressure on the pedal, always had to be gentle and ease it slowly.
Thats when I went to a stock 1967 Firebird Distribution Block, front brake proportion Valve and the rear Hold off It stopped locking up.
As far as Vacuum is concerned, I have a low vacuum cam. I run about 12-14" and I also have to wait for the booster to charge up, then it holds.
Are you sure your booster isn't leaking? or
Your Check valve is it working right?
I have something on testing out brake boosters a friend sent me last year, I'll see if I can post it here.