i have a 1998 mustang gt with a 5 speed manual tremic transmission. I would like to know if this is a stock or aftermarket part.
Depends on what type. It came stock with a T-45 which is a Tremec unit. Common swaps are are the TR-3650 TKO (uprated version of the TR-3650 found in 99-01 Cobras, late 01-04 GTs, and 03/04 Mach 1s) and the T-56 6 speed (stock in the 03-04 Cobras)