One thing you didn't mention was this transmission your talking about ever rebuilt, or just an old trans with a shift improver kit slapped in it? IMO A fresh properly built stock 727 with nothing more than a TF-2 kit should survive a long time. You do have an early model with the narrow front reaction shaft bushing but if the end play was set correctly I've seen them last a long time.
The 2 biggest things that make or break a trans in a HP application is how well it was built and does the car hook up. You could have 750 hp but if all the car will do is blow the tires off a stock trans would live behind it a long time
