I have an overdrive in my 55 Chevorlet Pick Up with 3.9:1 rear and 235 75 15 tires on 8" rim with Fenton headers, small body HEI And a Clifford manifold with a 390 CFM Holley and I have trouble holding 70 mph in overdrive. Can run 60/65 and hold more than 10 inches of vacuum but at 70 5 to 0 inches of vacuum. I have an dual 1 barrel manifold that I an thinking of installing instead of the Clifford 4 barrel manifold. Will be using Carter replacement carbs for the Rochesters which are now on my 48 Plymouth.
