If you shortened the engagement rod to its minimum length, does this loosen the belt tension when deck is engaged? If so, does it get even looser when disengaged? If you remove the rear tension spring, will the deck move forward to release belt tension? If these are both yes this is what I would try.
1. Double check that you have the correct spring, correct belt and it is routed properly on deck and stack pulley. Blades should turn easily with the deck disengaged.
2. Remove both engagement rod and tension spring. You should now be able to easily move the deck forward and backward with your hand.
3. Reattach the spring. You should now have decent tension on the belt, as the spring should be pulling the deck backwards. Not quite tight enough to run the deck, but tight enough that the blades will not turn easily.
4. Adjust the tensioner rod to its shortest possible length and install. When installed and the lever is in the disengaged position, the deck should move forward and up and all belt tension removed. When engaged there should be belt tension, but not enough to properly run the deck.
If everything is working properly at this point continue as follows. If not post back and let us know where problems occurred.
5. Now slowly apply more tension to the belt by making the tensioner rod longer. Check for proper engagement and disengagement after each change. I would go about an 1/8” at a time until I get the tension I like and the deck moves back and forth properly.
Hope this helps, wish I was there to see the problem, because I am making assumptions about guides, pulleys, guards, etc.
