This really isn't a job for a first timer. The ring and pinion gears are a matched set, and setting them up requires a few specialized tools. If you tried to keep the original pinion gear in the housing, its a bad idea. If its a different ratio, theres no way it will work. The tooth count on each ratio is different and they will not mesh. Also, POSI is a Chevrolet term. Mopars used the name Sure Grip.