215/70R15 @ 775 revs per mile = 3,700 @ 70 with 4.11 gears.

The 700-R4 would make it 2,600RPM @ 70, and a 3.07 rear with your current trans would make it about 2,800.

Another option would be an overdrive manual trans like the T-5.


Some of my crap:
1963 C-30 flatbed dually 292/SM420
1965 C-10 LWB fleet 250/column shift 3 speed
1965 C-30 9' stepside 250/SM420/4.10 gears (my DD)

Website I made for my crap:
http://www.66submarine.com