Go to this thread in Stove Bolt.com.
http://www.stovebolt.com/techtips/calculate_rpm_tire_axle.htm

It will tell you what changes you can make to the original drive line how they will will effect your set up.

In anycase 70 should be the best you can hope for.

My 1941 1 ton pick up has been set up with a small block 350, 700R4, 3.73 rearend and 17" tires. I expect 70 mph at 2,000 rpm.

My 1946 Suburban is getting a stock 235, 4 speed granny transmission out of my 1941 1 ton and a 3.43 rearend with 15" tires.

Both vehicles are getting disk brakes up front.

My first 46 1/2 ton got Patricks 3.55 rear end and it alone made a big difference, 60 mph became the comfortable speed.