Dunno if it's any help but I had the same thing happen on a little old Yammy when I was a teenager. I took them to a garage to get an airgun on the tops but that just shagged them even further. I finally got the threads to move by clamping the top as tightly as poss in a vice, loosening it up, turning the tubes 1/8th of a turn, reclamping, loosening, turning etc until I'd gone all the way round a couple of times. The clamping deformed the top of the tubes just enough to free up the threads a bit. Don't take it the wrong way mate but filling the forks down the side of the tubes sounds like a chronically bad idea. It's even a bit too bodgy for me!