Switch to manual brakes makes car run better?

Car is a 73 Duster with a 74 318 smogger with Summit K6900 cam, Performer intake, 600 Edelbrock carb, decent advance curve, and duals off the stock manifolds. Drivetrain is a 904 with stock converter and a 2.76 rear. It's obviously no race car.
278/288/112, and 204/214@050
on a ~ 7.8 Scr smogger, with a stock convertor and 2.76 gears.
How is your power below 3000rpm?
How you liking passing gear at 60 mph?