Yes, you can remove the rear of the 905 box. The limit is removing so much that the spring falls out, but there's no point going that far, as you still have the shank sticking out. I've shortened a lot of them on narrow gauge cars, and it's a lot less frustrating if you do the cutting after the box is screwed on. If it's welded together you can do it before, but if it comes apart, it's very hard to get back together.
I trim the back, on these and the 1025s, with a standard #11 blade, making several light cuts, rather than trying to chop the end of. If you do end up squeezing the box a little, and the coupler binds, pry the end back up a bit with the knife.