'63 is a different axle.
3.90:1 was stock.
I know you can cruise 65mph all day with a 235 & 3.90 rear axle because I drove To MA & back to OR several years ago. with no issues. Changed oil on the east coast before heading back. Still running same engine now, doesn't use oil. If it's in decent condition & valves are adjusted will not be an issue. Used to run 80 MPH with it back in high school (1970s) road racing a buddy with a Jeep wagoner hurricane 6.
