I added a 3 point hitch and PTO to my 1988 John Deere 350D. It is not hard, nor expensive if you go with used parts. I paid $268 for the parts for the PTO and fabricated my own 3 point hitch.
For the PTO, you just have to unbolt the rear end plate that is located under the fuel tank, and in front of the rear end. That cover has to come off, then you bolt on a new cover that has a winch drive with coupler roll pinned to it. You just slide the cover on with winch drive shaft until is slides onto the tail shaft of the transmission. You will want the plate super, super clean so the gasket-goop sticks and does not leak oil, but then bolt it on. That will give you a PTO with 21 splines.
To get it to 540 6 spline, you can either change one half of your yolks out on all your PTO shafts to 21 spline, OR, buy a $16 dollar 21 spline/6 spline adapter. The stub sticking out of the back cover is pretty short, so I took a 4-1/2 inch grinder and cut a flat spot on your 21 spline shaft so the set-screw for the adapter has something flat to screw onto, as well as not slip off from. I also added thread locker to the set screw so it would not back out.
