John:
I installed a 90 Dakota rearend with 3.55:1 gears and the engine is turning 3184 at 70 mph. The horsepower on these engines was taken at 3600 rpm and should be able to be maintained without hurting the engine. With the stock 3.9:1 rear end 55 mph was 2750 rpms. 3184 rpms gives you 63.75 mph with the 3.9 rearend. I would not think that 435 more substained rpm would damage the engine but I could be wrong.
