So those side rail pieces, as far as i can tell from the pics, are the EXACT same pieces used in the b219 subframe. The differences i can see are that yours have the plow brackets hanging off the front, whereas the b219 subframe has a large angle iron welded across the bottom right under the steering gearbox/pedal area, basically where your 2x4 already is. The uprights bolt down to that large angle. There are diagonal braces from the top of that angle, up to some empty holes in the 'trans tunnel' right underneath the steering gearbox. Those holes have plastic plugs in them (if they haven't rotted away). There's also a slight difference in the diagonal bracing between the 2 side rails, but i would say you already have the entire 'hard part' made, to oem spec, because it's the same piece the oem used for the loader! You would just add the large angle across the bottom for the loader towers, and the diagonal pieces from that angle to the empty bolt holes in your tractor.. those pieces are maybe 1/8" thick 4x12" or so, with 2 simple 45 degree bends you could make in a vise or shop press. Can be seen in the pic Lil Foot posted:
View attachment 96295
So you're like 80% of the way to having a factory b219 subframe already.