Frank,
Several folks have put 2" receivers on the back of their 400 series Power Tracs. If I was going to add weight, I'd consider doing that. First, a 2" receiver can be really, really handy back there VS the pin hitch. You can make adapters for trailer towing, ball hitch, pin hitch, ring hitch, etc... Second, you could make a 2" adapter that goes straight back a foot, then weld on a section of barbell rod up vertically. Those are easily found at any junk store, goodwill, salvation army, etc... and then you can throw weights on the barbell and secure them with the barbell lock collar. Piece of cake! :laughing:
Anyhow, I'm reluctant to add any weight to the back of mine. I use the pucker as an indicator that I'm about to stress the machine. I wonder how much weight those center sections can actually take.