Keep your '64's 4:10 rear and use a 700R4 instead of the Turbo 350, I did that in my '65 I had, the lower first gear in the 700R4 made it launch much harder and the overdrive made it EZ on gas when cruising.With new HEI's selling for around $100 junk the stocker and get a new one with a better performance curve and higher voltage to work with your cam and intake.