A smaller bore does create more pressure. Manual brakes shouldn't be a problem just don't forget your residual valves in the front and back lines (10 and 2 lbs respectively). I didn't notice if you were going to a dual master or two master's side by side, which is common in a race car.
By the way, I have a dual carb set up with a 3/4 cam and use a dual action fuel pump, dual diapraghm 7" booster, dual master and put a vaccum cannister in the brake line with a check valve and it all works fine. The vaccum cannister I got from Summit was $25. With the A/C dual action fuel pump there is plenty of vaccum and the dual diaphragm master requires less than a single to work.
'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi
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