It has been awhile, however this is my story. First I purchased a boom from one of the normal dealers. Then I ran into the same problem you are having now. At that point we returned the boom and called Barefoot International which made a clamp
that would fit with the correct clearance around the pylon with the motor box being closed. Well not as perfect fit as they said.
The solution we came up with is to make the brackets that off set the motor box 1" to the rear. (Modified "S" Hooks) On the four mounting points of the motor box, orginally screws with black knobs, make a loop on one end of a "S" hook and a post on the other. mounting screws go thru the loop with the other end being a post that will go into the spot where the screw orignally went thru. We also had to closely check the clearance at the front of the engine. 13 years and still having fun with it. Good luck....Curtis