When is a 904 too small?

I rebuilt a 904 6 yrs. and approx 12,000 hard miles ago using the Alto thin red's for my 3450 lb Cuda and it's holding up fine behind my 408 that's a torque monster and makes about 475 hp. I'm not even using a manual VB which provides more clutch pressure to reduce the chance of slippage. Just a stock VB with a TF-2 kit. The biggest problem I have is keeping back tires on it, lol... I can't say the exact limits to them but if you build them right they'll for sure take a whole lot of power