Damn dude, u have the same problem as i did with the same fork... Accept i couldn't even get screw it on. If you get it off I Recommend taking it to a TAP machine they have those at Machine Shops. what it does is it Re-Threads it. So im preety sure that would work
Put the nut in a vice, take your front wheel off, put a pipe (old bars, t-tube, whatever fits and is long) in between the fork legs, and crank it loose. If that doesn't work, get someone strong to do it.