The implement PTO shaft must be free to lengthen and contract to compensate for tractor moving over high (lengthen) and low (contract) points in the ground.
If the PTO shaft is NOT sliding reasonably free you need to soak it with spray "Blaster" solvent for a few days, then pull it apart, clean, remove any rust and lubricate it. Then check to see if it lengthens and contracts reasonably easy.
If the PTO shaft is sliding reasonably free and is still too long, the PTO shaft needs to be permanently shortened. See videos.
My problem is that the tractor does not have integral hydraulics so when I mounted a front end loader the only way I could operate it was with a PTO pump. Great but every time I wanted to use the pto [ tiller etc.] I had to pull the pump and block up the loader so it wouldn't just drag on the ground. I found a 9 inch pto shaft extension that slides over the existing shaft. Now I can leave the pump attached but the end of the pto shaft is too close to the implements to connect. I figured by lengthening the 3-pt arms I may gain the needed distance from the pto shaft to the implement.
Many years ago I see some draught arms that were in two parts with holes drilled at equal lengths along each part. The farmer would undo a couple of nuts and bolts, select what length he wanted and find two suitable holes to put the bolts back in. But as I said at the start, I`ve not seen any of these for years.