I've not taken one apart, I suspect you have. Sounds like the helical could have been misassembled- I had a Canon 35/1.8 with that problem.
Feel free to post pictures on this thread.
some questions:
1) How thick of a wedge did you use.? Would have to be 2mm or greater for a 50ft back-focus?
2) Will it focus to Infinity with the wedges?
to correct for a few inches, I use copper tape. For a big back-focus, I use a split retaining ring from an old parts lens. Like a namering. I also got a pipe cutter, and some aluminum. Cut the pipe, pilish the shim.