Best performance modification you could make is to stuff a TH 700 in that car. The 307 is similar to a 283--NO TORQUE. Coupled to a 3.08 rear axle, no cam change is going to make that thing move.
If you have a Potatoglide in the thing now...you won't recognize the car after you install an overdrive.
If you have a Potatoglide in the thing now...you won't recognize the car after you install an overdrive.