I've got a 268H in a F-150 with 3.25 gears and a C6 with a stock converter. It's just fine. My engine is no where near stock, but the power is there from idle to out past 5000. My C6 shifts when the tach hits about 5200 now. Because tachs lag a little, the engine is probably turning 5300-5350. Contrary to popular belief, you CAN adjust WOT shifts with an adjustable vacuum modulator. I did.