Any competent machine shop should be able to take care of shortening the shaft for you 3mm and extending the splines if needed.
Since I don't think I need any extension of the splines as I have a lot of contact area, I'm going to have a go at taking about 1/8" off myself. I have the chop saw, so it's worth a go. I can always back out of it if I screw up too badly and take it to a shop. Seems like something I'd like to be able to claim I was doing at the weekendI did on my rear axle shafts when I built it. Very easy process, just take your time and a little water will help. I cut both down by about 3/16".
I made sure to be really careful pulling the axles. The wheel-end seals and bearings are undamaged. The only one I might need to look at is the LHS inner - this can be done by doing a 'tilt' test, putting it back level, pulling the LHS shaft out a little and seeing if I got any fluid in the tubes. My guess is that it's actually OK, but we'll see. I was also able to test the full diff / ARB locker after all of this shenanigans and it's all functioning on spec, so I think (fingers crossed) I got away without damaging anything significant.I remember I had to pull one of my shafts because the bolt pattern from currie was not lined up right. I know the seals were trashed and I am pretty sure we had to get new bearings as well.
My guess with this is that if it has been a currie 3rd in the housing, they could have tested the mate of the system end-to-end before I picked it up, but since I got the 3rd separately sourced and delivered straight to me, there was no way to set the details of the fit until, well, this past weekend. (Makes you wonder how they ever got all of the bits on the space station to work right - but then my axle didn't cost like $100bn). Actually amazes me that all of this stuff actually did fit to within 3mm given that everything on the housing is custom fab'ed (welded) from the fraken-bits of the RockJock ends, the Currie fabricated center, etc.