The old axle center line is approximately 1/2" rearward of the pilot holes. It is 150.5 inches from the rear frame horn with the bumper extensions removed. There is a lot of wheel well opening on the front of the tire in the stock location when its lowered. Moving it ahead an inch will center it in the well as well as giving you tire clearence on turns.
It you don't have the front sheet metal clip off yet put a tire in the well, measure from the rear frame horn to the tire hub center. Remember that 55 2nd - 57 frames are different than 58 -59 frames. 55 2nd - 57 are shorter from the radiator mount forward.
Take a look at these two articles they may help you get an idea of setting it up. Its not a fatman but the idea is the same. This setup is much more difficult than a fatman and I don't advise doing it unless you have a lot of experience. The second article will help you check your frame before you begin.
http://www.webrodder.com/article.php?AID=264&SID=60
http://www.webrodder.com/article.php?AID=261&SID=60