The rear mount was trickier than the front. Again, I made it up as I went along and had to change it around a few times before I got a setup that I liked.
I used the bumper mounting holes to mount a 2x4 across the body. I used plywood spacers to get the board to clear the body near the center. Since the board mounted to the body doesn't have a vertical face, I cut a 2x4 into a wedge and used the wedge between the 2x4 and a 2x6 piece. The wedge holds the 2x6 so that it's face is vertical. (Some pictures show a 2x4 board instead of a 2x6 - I later changed it to a 2x6 to get a larger space to bolt the arc into it.) Long bolts sandwich the 3 pieces together, along with the cut-off piece of the wedge over the end of each bolt so that the washer and nut clamp down on a parallel surface. Does that make sense? Hopefully the pictures make it clearer. I also counterbored a recess for the head of each bolt and its washer so that the bolt head wouldn't stick out and risk damaging the paint or body.
The rear arc is then bolted to the vertical face 2x6. I put the bolts through from behind first, before bolting the wedged sandwich together for the final time. The arc then slides over the bolts and the washers and nuts are put on. For additional stability, I ran diagonal braces between the bottom of the arc and the frame rail. I think I attached the brace to the exhaust hanger holes in the frame rail. My car has a set of those "jack-up" rear shackles and it was easy to bolt the diagonal brace to those too.
Like the front arc, I positioned the center pivot point about 4 inches to the passenger side. It should match what ever you chose for the front.
Let me know if more detail or pictures are needed. The car is still on the roll-tisserie so I should be able to take more pictures or measurements.
