Use this calculator:
http://www.wallaceracing.com/calc-rpm.php
Your overdrive ratio is .70 assuming it has a 700R4. If you have 3.42 gears you could get away with running 33" tires and not doing a gear swap. If you have 3.08 gears you'll need to swap gears. You want at least 1700 rpm @ 70 mph. Or I would. Something between 1700-1900 would be pretty good. You wouldn't want the rpms so low that the R4 starts gear hunting, it's not the strongest tranny.