If you have adjustable upper control arms then just install them and use them to adjust pinion angle. The stock length may not be what your after. I usually install one and adjust the pinion angle then adjust the other side so the bolt slides in easily.
Some people use the upper controls arms to center the rear end and to add a little preload but I don't do that. I used my lowers to center the rear end and the uppers to simpl adjust pinion angle. I don't like adding preload in with the uppers.
Mark Thompson PSCA Mustang Maddness 37
84 Mustang GT, 408 super Vic intake, high port heads, pro system 1000cfm carb, inductions solutions sledge hammer nitrous system, glide. Full TeamZ suspension.