If your existing brakes and axles are good, you can use the 3rd member out of a 55 or later, keep all of your existing housing & springs and keep it almost stock. Otherwise, you may want a newer rearend, such as Blazer because the 55-59 only get you to a 3.90 ratio and the replacement axles are hard to get, shoud you need one.
Because yours is a 45 , I don't think you need to do any spring perch relocation. I didn't on my 46 Chevy & it looks just fine. However, there are some subtle differences between GMC and Chevy, but I don't think this is one of them.
