727 or 904?

If you change from 904 to 727, you'll have to get a shorter driveshaft with a different slip yoke and a different converter and possibly flexplate. 727 is naturally stronger than a 904, but a 904 can handle a lot of HP with basic HD/performance upgrades and will be about .15 faster in the 1/4 mile. 904s also don't have a tendency of blowing up if the sprag goes out.

The 727 should get upgraded also. A good shift kit or standard forward performance valve body will go a long way to minimize heat, slippage and prolong its life. A cooler is a good idea for any performance application.