Keep an eye on ebay. They come up often. A brace for a GL1100 will work, but needs to be shimmed slightly. That's what I used on my '78. You just missed an auction one of our members here had on ebay for a brace that would have worked well for you. Went for a very good price ($33!).
Yes, slightly. I don't know exactly how much, but I believe I used 40 mil shim stock. I originally tried aluminum soda cans, but they didn't hold well. Good shim stock and it's solid as a rock. You'll see these braces on ebay ($100 and more new) with shims included.