A wood lathe is not rocket science,
take two Channel iron, or I beam for the bed, cut spacers and weld them in the center with a gap between the two channels or I beams, In MY opinion if the beams have narrow flanges most likely the better,
put that on a stand build a head stock (shaft set with two pillow block bearings,
have a key way cut in the shaft for a pulley or step pulleys, have one end threaded and or have a machine shop drill a hole in it or use some bushing stock tubing, and have a mortise taper reamed in to the end of the stock, (your going to need some type of drive, if threaded one could use a commercial face plate but if you get the taper put in one can use manufactured live center,
on the tail stock one will need to make so the center is the same hight as the head stock,
this could be a piece of tubing or nearly any thing that you can weld up, fix a flat to keep it aligned on the bed, and then a hole for some type of bolt, to lock it down,
to make the tail stock so one can adjust it, my thought has all ways been to use a large thread shaft and then weld two nuts on the tail stock you made, put a hand wheel on the tail end and (if it was me I would have a hole drilled into and ream a mortise taper in the front end, so one could use a commercial made centers, but if not even having the center turned into the threaded shaft, take a third nut to use as a locking nut, this way one can adjust the tail stock by turning the threaded shaft when correct tighten up the the third nut to lock the shaft in place,
here is form with a home made lathe some what to what I described, the bed is a little different, but the head stock and tail stock is similar,
http://www.woodworkingtalk.com/f12/homemade-lathe-3037/ about half way down, the page,
if your doing large items one may have to make special centers to hold the work correctly and safely,
I do not think that making the lathe is a poor idea, I do think that powering it with the tractor with out any way to quickly shut it down is and not limiting the power to the unit,
I made a small lath when I was in high school with some 3 inch channel iron and it worked well, (it would have have been much better if I would have had the shaft turned and a Mortise taper put in, as I had crude home made centers, (if having the taper cut is not possible, one could have a "Solid Drill Sockets" pressed into the shafts and lock tighted, (there mortise taper and hardened, and a round and straight out side),
one can buy a reamer or a scocket from ENCO or MSC, travelers and Wholesale tool,
or an other of the machine tool supply places,
(this is jsut an example
Solid Drill Sockets (Hardened & Ground -- WT Import))
