Mine are part # 5350 and were purchased back in 2004. Not sure if the arms have changed since then. My instructions read;
"7. With the stock bracket removed measure back horizontally 10.25" from center of the stock bracket-mounting hole. (The threaded 1/2" bolt hole on the outside frame.) This marks the position of the center of the 1" hole in the new bracket."
"8. Based off the measurement in step 7 slide the frame-mounting bracket into place. Mark the three mounting holes for drilling. Remove the bracket and drill the three mounting holes in the frame. It's best to start with a 1/4" pilot drill and then progess up to the 1/2" size. Drill these holes all the way through the frame. CAUTION: Watch for brake lines, fuel lines and other possible items that may be on the other side of the frame."
James Duff Instructions #5350C 11/03
Probably best to check with Duff in case things have changed. If your having a problem making contact or can't wait to Monday I can scan and send a PM with what I have.