Help fine new ratio

Hi We have a 1971 challenger, 499, 727, 4.30 rear gears. the car did 10.32 @ 129mph. Ok so we are in the middle of rebuilding the car for the road what ratio would we need to go to if we wanted to cruse the hiway at 70-75 @3000 rpm or a little under? Oh yeah we have 29.5x10.5w R15 tires on it now and will run a like tire on the street. thanks
Author: motasupra