I finished the install of the wheel spacers Sunday afternoon. These are the ones from the Ebay store, here's the link again. They have 9/16" studs, very close to the 14mm size the 3720 tractor came with.
Skid Steer Wheel Spacers 8 Lug Red 9/16" Skidsteer - eBay (item 320476519394 end time Aug-15-10 21:52:22 PDT)
Had help from 2 nephews, so I had plenty of time to take pictures while they did all the heavy lifting.
Fit was excellent. The centering features worked fine with both the tractor hub, and the rim. The male feature on the spacer could have been a bit longer, to penetrate the hole in the rim further, but it is certainly good enough. It would be even better if I flipped my rims back to the narrow position. I may do that, but will run it this way for a while first.
One dissapointment was the fact we had to remove the wheel weights to be able to put a socket on the nuts to hold on the rim. I had machined the ID of the inner weight to make socket clearance for the original 22mm screws, but since the nuts that came with the spacers were 1" wrench size, the proper socket was way bigger than the 22mm socket for the stock screws. Oh well, good exercise.
I locktited everything with blue 242. And torqued everything to 140 foot lbs. I used both 2" spacers on each side, for a total of four inches per side. Overall width from outer edge of tread went from about 59 inches to about 67 inches.
I did not have time to go to the steepest part of my property to brush mow for a proper test, but I did mow the lawn and immediately noticed a change. The tractor used to "jerk" side to side as I'd drive, as one wheel or the other would fall into an irregularity, and the side to side motion was very noticeable. That effect is gone. It feels like it rides much flatter on the terrain.
Pics attached...