Saluki, looking at Partstree, there should be 3 washers that keep the double pulley spaced off of the deck. Maybe they are missing?
I'm thinking the idler bracket bolt should not be tightened up. It should be a flange lock nut on the bottom to keep it from falling off so it doesn't need to be tight. I would loosen it up some so that the bracket can move.
Make sure all your pulleys turn free also. Here is a deck belt about 1/2 the cost of an OEM, might be worth a try.
http://www.ebay.com/itm/MTD-or-CUB-C...YAAOSw0QFXC6WH
Here is the Partstree look up if you need it.
http://www.partstree.com/parts/cub-c...-cutting-deck/