Installing a 915 trans from a 73 in an 82 SC

What is involved in replacing the transmission in my 82 SC with one from a 73 911T (with the 7:31 r & p)? Is it a direct replacement? Or will I have to add/subtract components? I was looking at getting my trans rebuilt with the 7:31, but I found a guy that will sell me the trans (rebuilt 20K ago)from his 73 at a great price. Which way should I go? Thanks in advance.