The bronze clutch throw out button should be tight to keep from turning. It should be made of oil impregnated bronze to keep from wearing. Both belt numbers referenced in thread are the same length. One is a 1/2" width and the other is 3/8". Either width will work. The one you already have is the 1/2" width and thus needs to be at least a 4L820 or 4L830 (82" and 83" respectively). From the looks of the pics I would get a 4L820 if your current belt is not jumping off the pulleys.

According to this chart, there is a second belt for earlier decks that is one inch longer.

Also, that spring might be to long (and too weak).
This is a spare NFQA frame that I have...

One is 80" and the udder is 81", both are 3/8" Had the same problem on my 86.

Ironman, where did you find that chart? All cross reference charts and internet sites that I can find show the 490489 belt as a 1/2" x 81" or 1/2" x 81 1/2".
I use a 1/2" x 82" and my idler pulleys have belt going onto and coming off of belt parallel with clutch sheave.

68IH105 if you straighten the idler pulley bracket and get a shorter tension spring (or cut current one shorter), then decide if you want to run a 3/8" or 1/2" belt. If you opt for the 1/2" belt pick up a 82" and a 83" and use the one that aligns the belt parallel with clutch sheave. It's going to be trial and error because of all the wear on your frame and mount and what appears to be mismatched parts.
Also, is the frame to mount assembly pivot rod in the front hole of frame. If not you can put it in front hole and probably use the belt you have.

Here ya go....
https://s3.us-east-2.amazonaws.com/c...+1947-2004.pdf

I wouldn't do that. The angle keeps the belt parallel going back from the idler pulleys to the deck pulley.
My spare frame is angled the same way.

Thanks for the link Ironman. BUT, that is a CUB CADET CORPORATION publication and we know there has been numerous errors pertaining to IH manufactured equipment published by CCC. All pictures (not drawings) of that hook shaped idler support show it NOT bent. If you think about it, when bent in that direction it actually sends more belt twist to deck pulley than it would if it were perpendicular to the shaft.

OK, they got the publication wrong, and they are selling the wrong product too.
Note: says 3/8" not 1/2"
Note: says OEM, not, "a replacement for"