looking at the thread on the end of your shifter, it's looking a little sad, so my guess would just be that the matching thread on the gearbox housing is just equally worn/stripped over the years to match, which might be why the previous owners had to use wire somehow to keep the 2 screwed together of.
I think you can get the thread plate part of the gearstick, which would help 1/2 of the problem.
Depending on how far you want to go, a bodge might be to try to use some of the 'JB Weld" type products on the threads and refit?
I have a tube of the stuff, but have never used it yet.
edit: Marty just said your threads were cactus also..