If you got a Buick V6 professionally race balanced and installed a different cam, I'll bet you could get 6,000 rpm out of it. ;^)
Remember that while an overdrive increases road speed at a given engine rpm, it decreases torque at the rear wheels. The factory overdrive (fifth gear) in my 2.5 liter Wrangler is pretty much useless for that reason. So if you install an overdrive or regear, you will probably need to increase engine displacement to compensate.
Earle