I'm at 405 hp at the flywheel and so far I have had good luck with a Centerforce Dual Friction clutch. What I like most is the low pedal effort and the fact that it holds so much better than a stocker.
Centerforce has a new DYAD
clutch you might want to look into.
also makes good clutches, so stop by there too.
There are many more clutch manufacturers out there, so if either of those don't suit, just Google Mustang Clutches
for more choices.