Skip the flexplate, unless you are separating the engine/trans to do the rear cover on the motor of course.

If you can't get 200K out of a GP then you either have a lemon, a basketcase or just treat cars like shyte. My 95 Bonneville is sitting at 311K (original motor and trans) and my 00 GP GT just rolled 427K, with 377K on the motor and roughly 320K on the trans.