Need correct starter info for 1991 Mustang LX

Hi everybody -
I need help identifying the correct starter info for a 91LX with a 331 stroker and Tremec 3550. I inherited the problem when I recently bought the car. Car actually starts (and is wicked fast), but squeals / grinds for 3-5 seconds on cranking. Then runs beautifully. The existing starter is a Ford Racing low profile, hi torque unit and the teeth are definitely damaged. Previous owner is 100% certain the transmission and bellhousing are correct, as they were re-used when he rebuilt the engine about 500 miles before I bought it. However, he changed starters because he wanted more clearance with the racing headers and 3" exhaust. There never was a problem with the other starter, he just wanted to upgrade. I'm thinking the starter that is installed is for an automatic - but I can't even find a part # or model number on it - so i don't even know what I really have. Is there anyone out there that can tell me exactly what I need???? THANKS!