I can often clean lightly gunked threads with a sharp utility knife. Start at the second full revolution in and run the blade counterclockwise out of the hole, lightly at first. The steel blade will cut a good enough thread in the aluminum.
Another option is to make a thread chaser with the body of a broken crank puller--cut two notches into the threads 90 degrees apart. It's a crappy die, but it often works.
If all else fails, use an automotive ball joint separator, a "pickle fork." It might run the crank or dent the BB shell, so it's a last resort. There are also machined wedge sets that have been shown in other threads on the subject.
Good luck with it. I rebuilt an '83 520 a couple years ago and I love it.