Rear gear recommendation for highway.....

67 valiant post with built 318 and 8 3/4 3:91 sure grip, 727 tranny. I recently bought the car and would like to be able to take this thing on the interstate sometimes and cruise at 65-70 without the rpms being crazy high. Recommendation for a higher gear for this combo? As it sits right now if i even do 55-60mph its pushing 3K and higher......