My old 6 ft finish mower has chains and a fixed top link pin to the implement has worked great for 15 yrs,
You are getting warm. My comments are:
1) The OP's top link position (as another guy said) is wierd. Not sure what the issue (odd tractor ? Not SAE std 3pt dimensions? ) is but you need to use a lower hole for the top link attach point on the tractor. A previous post said that made it worse.
It won't after the rest of the problem is fixed. I could be fooled by the pictures but pay attention to this: Y
our goal should be to have the top link closer to the same angle relative to the ground as your PTO drive shaft when in normal flat mowing position. (That's regardless of which top link attach hole you use.) Don't have to match but needs to be closer. From side view it ought to look like a parallelogram moving up and down.
2) When I say 3gunr is getting warm, I too run chains on a bush hog from the A-frame back to the rear of the hog. However, looking at this finish mower I think you want to retain the stiff structure above the deck of the mower. To do that just use a
single chain between top of the fixed A-frame (getting rid of the original swinging clevis) and the tractor top link attach point, getting rid of the stiff top link. The
combo of using chain there and choosing the top link attach point solves your problems. The chain allows all the freedom you need for the mower to go high crossing a ditch, etc. As you said you DO have to be careful not to put the PTO shaft/u-joint at too bad of an angle but you, as operator, should/can control that. Don't expect the hitch-up configuration to always prevent it. Choosing the top link attach point for best angles on the tractor [closer to parallel with the drive line] also reduces the variation in U-joint angle as you raise and lower the mower.
The chain length replacing the top link becomes your adjustment for the comproimise between allowing adequate up down motion of the rear of the mower crossing ditches and humps versus being able to pick up the mower as desired.
I think you will be AOK with my suggested configuration above.