So because several members here said that it was a direct swap, I've been collecting '75-'76 GL1000 parts for the past few months; refinishing, rebuilding, restoring; bringing everything up to first class. Several hundred dollars and dozens of hours of blood, sweat and tears later, I had everything I needed. Was a bit concerned that the fork legs were about three inches longer than the stock legs, but figured I had a way around it, so I started the project this morning.
Long story short, the steering stem isn't even close to fitting. It's at least an inch longer than the CB500 stem, which means the two threaded areas and the area where the top race press fits are in the wrong spot. It may be possible to find someone with a lathe to continue the threads and turn down the press fit area, but then again it may not. Certainly this has turned into a much more complicated project than I had believed, and I am pretty damn pissed at the moment.
Now, I'm hoping that I've just missed something, and I'm hoping that someone around here who claims that this swap works will arrive to explain to me what I need to do.