How much Vacuum?

15 usually is enough. if the pedal is spongy you may need to bleed the brakes or swap the master cylinder. a power brake booster won't appear "spongy"

don't know what the minimum required vacuum should be, but 15 psi is a normal good engine reading and the booster should function well with that .... "spongey" to me always indicated air in the lines some palce or a defective booster .... the booster in mine doesn't provide the same feel as modern power disks .... so perhaps it is a relative thing .... do the brakes work well?? .... I replaced all of the lines and rebuilt the calipers with staniless pistons (from SSBC) and cleaned-up the booster air vent .... they operate smooth now but are touchy - very little pedal effort required to throw you into the w/s .... don't know if this helps ....

I have no problems with 13" of vacuum at idle and P/B. You should be fine. Prolly air in the lines.

My 67 has 12 inches of vacuum at idle due to the cam, but is enough to run the power booster. If you have those Kelsey Hayes type four piston calipers the brakes will be touchy and grabby.