Oh and to reply to the original post, to stay on topic here, I just ran into the same problem with a 1970 impala w/a 502 .
Pretty much the same setup too, gm front disc and calipers, wilwood rear calipers and rotor kits on a 9 inch ford rear ( Yeah..how bout it?)
dual diaphram 7 " booster, new master etc etc etc.
Seems the biggest problem was the same , inability to apply brakes hard enough to lock them up.
Turns out the pushrod angle on the brake pedal was not at the optimum 90 degrees when pressure applied was sufficient to stop the car/slide the tires.
anything more or less than 90 degrees will diminish the ability to apply force as it changes the pivot point or fulcrum which.
Seems the original installer had to mod something under the dash and ended up with a pushrod that was too short. It was about 1 1/2 inches too short.
Once that was fixed the car would skid the tires if desired.
The 7 in dual diaphrams work pretty well. We checked the vacuum supply and it seemed low (14 IIRC) due to the radicalness of the cam,but the booster was not the issue.
Dean oshiros article also clears up the power brake-non power brake master cylinder truths--myths
whatever you want to call it. It points out the facts like master to slave (w/cyl or caliper) bore size to bore size ratio importance , application requirements for drum or disc, etc etc etc etc etc etc .