I think can help with this. I've done quite a few of these as they are the same as on TY250A,B,C,D twinshocks. Yes there are 3/8 drive sockets that will fit inside the fin holes, but sometimes no matter what you do in preparation to loosen things up, there will be too much force required to undo the nut (with a high quality single hex socket, it just shears off the top of the nut)
To get the barrell off, a simple tubular steel drill guide that fits neatly into the fin holes will guide a drill so that you can drill the stud away to flange level without risk of damage to the cylinder.