Unless your 336 is a .45-70, which has a bulge in the left rear wall of the mag tubes to allow loading, they're most easily shortened by hacksawing what you don't want from the read end and sliding the tube rearward.
If the front of the original mag tube is retained via a stud into a bottom barrel dovetail, I wouldn't do it - since there would then be an exposed female dovetail to fill, etc.
If, however, the front of the mag is retained by a barrel band, a new half cross-slot can be filed in the bottom of the barrel, where the barrel band cross-screw moved to. The old half cross-slot, now exposed, will be hardly noticeable - or may be covered by a sling swivel band, as below.
If, for some reason, the mag tube moves forward upon firing, the movement can be blocked by installing an Uncle Mike's type clamp-on barrel band Q.D. sling swivel stud so that it bears against the end of the mag tube.
