What are the major differences between A904 and A727?

Just in case it matters, the 727 sucks up 20 HP more than the 904 because of the larger rotating parts according to Car Craft dyno tests. The 904 can easily be built to handle over 300 HP with the current parts out in the market. I see no problems from my slant 6 experience with it handling what you want and would also benefit you as it weighs about 30 lbs less.

Lastly, they are a little more difficult to find, require a new driveline and linkages. Just more $$.

Just my $.02 worth.