You could try restricting the vacuum to the booster... cut up a few appropriate-sized bolts and use the non-threaded area to put into the vacuum line. Drill a different-size restriction hole into each one and see how it affects pedal feel.
If you really wanted to get fancy, you could drill & tap just one to accomodate Holley jets and try out several different sizes.
If you really wanted to get fancy, you could drill & tap just one to accomodate Holley jets and try out several different sizes.