[MRGTO2U]

I hate to ask because I know so many people have asked the same but here are my specs. 68 GTO with a 428 headers, Carter AFB, Holley Dominator intake, Roller Rockers, Ultradyne Cam(223 intake/232exaust). Turbo 400 Tranny (Mild Shift Kit). Car now has a 2.78 open rear(I can't understand why previous owner did that).
QUESTION: I am buying a Eaton or Auburn Pro Posi Unit. CONSIDERING MY ENGINE AND TRANNY SPECS, Which gear would be better for acceleration 3.55 or 3.90? I know highway driving hurts with the 3.90 but my question is acceleration.

Thanks to all!!!