I have a 72 SS 402 with a Turbo 400. I have a 12 bolt rear end but it is a 2.73 open. I am wanting to convert to a posi-trac. Can this be done in the 12 bolt housing with new gears or do I have to find the whole axle. Also, what are the ratios for the posi-tracs and what axle codes should I look for.