Tires / diff ratio advise

A Body 440 WHY NOT said
The engine has not been on a dyno. Just using the cam as a guide.

Very inaccurate. Forget all that.

IN your case what I would do is just figure what RPM you'd like it to be at when at cruise speed. I.e. My car at 60mph turns 2500RPM with 28" tall tires, 4.56 gears and the overdrive on.