Ok , lets start with your converter . What stall do you have ? We used a 3000 rpm stall that set up tight . It would allow the motor idle at 750 rpm and move it with out much slippage at 60 mph , this was about 3000 rpm . The rear gearing was 4:10 with 32 inch tall tires . Did you change to colder plugs in the motor ? The Hemi in the cuda has a boss hogg converter with 2400 - 2800 stall with a anti ballooning plate in it , rear gears will be 3:73 with 32 tall tires , this should get me about 72 mph at 3000 rpm . What is your set up in your car ?