With that motor, I would modify or change the entire drivetrain. A lot of it depends on what you want to do. If you want to tow, work and get more top speed - A newer sm420 w/O.D. If you want a nice cruiser and more top speed - a T-5 and then maybe a higher geared rear end either way you go. DEFINATELY radial tires. But that's just me. I am biased and I don't really know squat
