I don't know the original booster diameter.
to help me visulize your question, I tried to make a graph that shows the amount of extra force that a 7 or 8 inch booster would provide at different manifold vacuums.
at 8" of vacuum, the differance between the 7 and 8" would be about 10 pounds force at the pedal. neither one of them provides much assistance (about 40 pounds).
so going from the 8 to 7" booster, to get the same braking with 8 inHG manifold vacuum, you would have to push 10 pounds harder on the pedal. IF the manifold vacuum were to increase from 8 to 11 inHG, you would not notice the differance between the 8" to 7" booster
force at the pedal assumes 4.3:1 brake pedal ratio
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