Larry, when you have a Berg 5-speed built you should select gear ratios and ring/pinion to give you the final drive ratio and gear "stack" for the performance you want. Going from a 4.12 to a 3.88 ring/pinion and shuffling the gear ratios should work fairly well.
Cooling is a function of fan output volume, integrity of the engine tin, correct air/fuel mixture, properly sized exhaust, power output, and oil cooling system(s).
A good final drive ratio in top gear is about 3.5 to 1 overall which with 25.5" tires will give around 70 mph at 3,250 RPM.