[bmxmon]

I would highly doubt it. If the gears in the back are to high your engine will be revving high at relatively low speed, opposite if your gears are too low. Do you mean its a 9 inch rear end? Pretty rare in 66 (like you said shelby only i believe), plus they didnt have 428's yet. (at least in the stang) Might be that crazy cam isnt the best thing for your little 289. Does it idle nicely, or is it real rough until it gets into the higher RMP's? Manual or auto transmission? Reason im thinking that is maybe its slipping and your not getting all the power to the rear wheels, though if your engine sounds like crap its probably that. Does it spin in gravel, if not then i would guess your trans is a bit messed up. Check for vaccum leaks too, they can suck. How old is the carb, might be time for a rebuild or just a new one.