Just a thought here... It is easier to swap the transmission than it is to swap the rear end. You don't mention what engine or transmission you have, but I'll assume you have a sm420 granny low 4 speed. Changing to a 4 speed OD transmission like the my6 (a chevy version of the A833) or even an NV4500 5 speed will give you an effective final drive ratio of 3.34:1.
Anyway if you had 29 inch tall tires, and the 27% OD either of those transmissions offer, 70 mph would be about 2700rpm. instead of 3700 rpm without the OD.
