Find a shop that has a modern Mustang dyno and you post more accurate numbers. You want a dyno that makes resistance to help spool up the turbo. The dyno I ran my truck on plugged into the diagnostic port and gave a detailed printout.
I'd guess 375 HP. I made 447. 5 HP with my HO but I also have a Smarty, High Tech Turbo Stainless Steel 62/13, BD manifold, and digital meth. My Smarty on level 9 (Comp box off and no meth) was enough to push my OEM clutch past it's limits (17 - 18K in tow).
I have a South Bend Con Ofe rated at 450 HP now. I think the clutch and install set me back around $1200. The drawback of a hefty clutch is it takes about three seconds for it to stop spinning when you push down on the clutch pedal. The OEM clutch stops spinnings almost instantly. This means that if your at a stop light in neutral and you push in the clutch when the light turns green your not going any where for three seconds. Get used to keeping the truck in gear at stop lights once you get your clutch upgraded.