Have a tremec 3550 I need rebuilt for my 95, anyone know of any good rebuild shops in So-Cal or any places I can purchase general rebuild and hard parts from?
I don't know where in SoCal you are, but I have had several people recommend Tustin Transmission. They are right at the 5 Freeway, just off Newport Ave. in Tustin.
As far as parts, I have read several articles on rebuild kits from D&D Performance - http://www.ddperformance.com/Tremec Parts.htm for high performance kits. Of course, there is always Summit or Jegs as well.
Speaking from experience, DO NOT let your "regular" Mustang shop farm out your transmission rebuild. It drove me nuts not to be able to talk to the guys doing the work, both during and after the rebuild.