So, here is where I am so far.
The answer to my question about the distance between the columns seems pretty obvious in retrospect. It depends on how wide the tractor is. For me, that was determined by the lights. The lift arms need to be wider that that, so I went 25 3/4" between the lift arms.
I started by replacing the support bracket under the engine with one that was wide enough the support the substructure but would still allow the wheels to turn. I had to drill two new holes for the engine bolts. Of course they are not in line with the original two, and not equal distance from the center. The heads of the 2 original bolts also are under the engine and spin freely. I ended up cutting a slot in the bottom of the bolt & holding them with a screw driver while turning the nut. You also have to jack up the front end & remove the axle pivot bolt so the bracket will drop down. The left side engine bolt had a large square head on it, so it didn't rotate. The right side engine bolt didn't, so i built one to replace the original bolt. It is ugly, but it doesn't rotate.
The substructure is 2 x 2 (1/4" wall) angles iron. It is supported at the rear by a 1 1/2" round bar that fits through pre-drilled holes in the frame. On the right side, it will fit between the wheel and the break rod, but just. There is some interference with the drive belt when the brake is applied, but none that wasn't already being created by the brake pivot.
On the left, it will just clear the wheel. I might build some spacers to give the tires more room, or I might cut off some of the angle iron where it runs passed the wheels. Not sure yet.
The substructure isn't wide enough on its own to support the uprights and clear the light so I added some bump outs using 2 x 2 (1/8 wall) angle iron. That wouldn't hold the load so I added a crossbar under the substructure to carry the weight.
I built the uprights out of 2 x 4 box (1/8" wall). I welded the uprights to a piece a 1/4" plate so I could use bolts to disassemble unit for storage. The left side is taller so I have a place to mount the control levers. The HB112 has a hydro drive, and the lever is on the right. I put the bucket controls on the left so I can move & lift at the same time. Can't move, lift, and turn at the same time, but you can't have everything.
The lift arms are built out of 1 1/2 x 3 (1/8" wall) box. I cut an 8' length in half to get two arms. The location of the bend depended on the front wheels, and the length of the hydraulic lift cylinder. If the long section is to short, the cylinder would be shorter and that restricts the lift height. The angle depends on the height of the bucket attachment point and the height of the front wheels. If the angle isn't right you can't get low enough on the bucket to get a good tilt action without hitting the tires. I set mine by using a 4 x 4.
Just some things to think about if you are heading down this road. I still have a way to go. I still need to finish the welding, add some braces, build the counterweight box for the back, and figure out the hydraulics.