new brake system hard stopping

1970 Satellite, was drums all the way around, originally power brakes. Put SSB disc brake kit on fronts and changed rear to 11 " drums, all new. SSB kit included new booster, master cyl. and hardware. Brake pedal solid and car stops but only if pressing extra hard on brake pedal. Feels like no booster output. Engine is 426 Hemi with comp cam 543 lift, 292 duration hydraulic. Could this be a low vacuum problem, didn't think that cam would have too much effect on vacuum pull. What should the vauum reading be to have the system operate effectively?? Thanks

I have had good results running power brakes with 10" vac. My current running car has 10-11" at idle and drops to about 6-8" in gear. Brakes seem to work fine.

Now I just had a thought. Do you have one of those small diameter boosters? Could the lack of surface area be more sensitive to higher pressures (meaning not enough pressure differential to make it work?). You might need an accumulator.