Couple of thoughts from someone who has used one of these a LOT ;-)
I was looking at the tractor supply catalog today and they have most of the parts. I know that northern has a weldable wedge...
As far as the crossways across the back of the tractor, I would think twice about this. In my case, I would extend the third link to set the wedge end on the ground or on a small (~ 8") tall stub so I did not have to lift the wood up so high.
BTW - If you want to consider buying, my FIL has a HD homemade horizontal for sale now - PM me if you want more info...
FWIW - FaC