Will 2002 mustang transmission fit 1998 both 3.8l?

No, a 2002 Mustang transmission will likely not fit a 1998 Mustang, even if both are 3.8L. While both cars used 3.8L engines, the transmissions changed significantly between those model years. The bolt patterns, bellhousing, and even the internal components are different enough to prevent direct interchangeability. You'd need a transmission specifically for the 1998 3.8L Mustang.