Considered a Dana 60 full floater? The D60 has the same 9.75" ring gear as the Ford/Sterling 9.75 that comes in these trucks, but has tons more aftermarket support! I don't see why you couldn't build one to be stout enough for your power goals... NASCAR runs them with 800hp for up to 600 miles at a time and rarely do they fail.
EDIT:
Dana 60 axles are rated to carry more weight than the Ford 9.75" axle as well, so that would be a step in the right direction as well.