I'd recommend staying with the 8.8 rear, as they are good enough to take you past 1000HP when setup properly... cheaper than it would be to build a 9" combo that doesn't take as much HP to drive as a stock type 9"... (i.e. fabbed 9" housing, gun drilled axles, all the trick stuff,etc.$$$)
If you plan on going upwards of 550-600hp and plan on torturing it on the dragstrip, then the only way to go is 33 spline axles, a spool, and c-clip eliminators or 9" housing ends as Andy pointed out... I had problems with leaky c-clips using the paper gaskets, but have lessend the problem by using a bead of RTV silicone and letting it 'skin' before bolting the axles up
Works great so far!
...here's the setup I'll be using to get me deep into the 9's
click here for a current shot,
and...
click here for a shot from a few weeks ago.
-L8R!