Shouldn't that spring hook between the rear idler pulley bracket and to the rh frame rail? Some have a 5/16 bolt in the rail, some just hook thru the side.

Does anyone have a picture of the bottom (above the mower deck) of one of these tractors?

No pic, but what you did wrong is hook the spring for the rear tensioner to that front bell crank were it doesn't belong. It hooks to the side rail of the frame. Most of the lts hooked to the frame from the rear tensioner bracket.

tomplum, should the spring be like the picture at the link below, I drew a red line where i thought you meant to hook into the rh frame.

Here is a link that might be useful: Picture

The ones that hook through the side of the frame will hook through an area where 2 holes are stamped. Yours could very well have had a double nutted 5/16" bolt through the bottom of the frame rail to which it hooks. There should be mars in the paint that would indicate where. Some of the larger ones go left, but I'm pretty sure yours goes to the right side.