Captain Max Silver
I Had To Use 4.2L Flywheel And Harmonic Balancer Because They Are Different. 4.2L Internal Balance. Was To Bad Because I Have Several P/P And Stage 3 Clutch Assemblies. But They'll Find A Home.1999-early 2000 3.8 had external balance engines, with a counterwieght on the harmonic balancer, and a flywheel balanced for each motor.
It best to keep all three together.
Not sure about the 4.2, because it was never stock in a mustang. Sometimes flywheels can be put on different engines, like a 1967 289 I had, with a 11" 351 flywheel.
It was internally balanced, so I could use the 289 balancer.
I guess you have to buy all three pats from the same dealer, or if used, make sure all three came from the same engine.