It is a great machine! I should add that I do pull hte pump belt off to ease the load and keep the hours off that component when simply setup to mow. I also beleive a 42" deck would work fine with no modification, and Bob says that's what he used on his 3112H/515 combo.
Michael, I'd love to see the frame...I've considered that as well, but never considered the ease of a wooden structure. That might be the ticket.
You'll see me testing out the loader on the frame in this photo. Really easy to make and makes taking the loader off in a snap. I might have some improvements. First, I'd make the frame maybe 6" longer and I might add an upper frame which is wider than the front tires which you can hoist the frame up with some ratchet straps. The frame must be lifted just enough to drive the front wheels under. Pretty easy to make. My loader is sitting on that frame now. Drive up and connect it.