This tech article shows how to install an aftermarket differential in your rear axle:

http://www.mc2racing.com/tech/20081104a/index.html

Keep in mind the article is focused on the installation of an Eaton clutch-type or a Detroit helical-type differential into an open 7.5" or Cosworth 7.5" rear axle, but the concepts are all the same. At the very least you should notice that you don't have to mess with the pinion stuff at all. Installing a differential in your rear axle is actually not very difficult.