ok got everything back on and I definitely have the wrong belt. I know I ordered a cheap replacement for what should be the correct belt IH-487043-R1 on ebay:
http://www.ebay.com/itm/Cub-Cadet-47...item2c6953ffa9
3/8" x 28" but the 28" is putting me right at the end of the gearbox frame slider slots. The only reason I as able to get any tightness was because I think it started bending towards the thrower and angling the pulley, not good obviously. I'm going to raise the gearbox up a bit, use some string to measure and get a better idea of what belt to order and go from there.
I wonder why the "correct" belt isn't working though? I have a 4" pulley on there. Even with the hodgepodge of cc36/qa36 parts, it's still the same belt. Is there anything I could be missing?