If you can find a 50's or 60's truck with a New Process 541 with a "working 5th", they will cruise at 75 at 4200 RPM. I drove one for almost 5 years with a GMC 370V8. 4th and 5th is close ratio and with the correct 2 speed, you have less than 400 RPM split on your top 4 gears. I ran circles around 671 Detroits with it. Here is the shift pattern.

https://get.google.com/albumarchive...hKey=CL-Oxam8_7HplgE#6409303494956954690