Same here. Just cut the rear of the tube the same amount I removed from the barrel. I had to use a round file to create the recess in the bottom of the barrel for the tube clamp. Other than that, no problems.
Ideally, measure the length of mag tube for the number of cartridges in your mag, and figure the "excess" as spring clearance. Based on that capacity/spring clearance calculation, determine how much mag tube to chop.
You can of course, cut simply on the 16.5" desired length, but you might find that a 16" mag tube would be adequate (save a bit of weight) or that you might keep one more round with a 17" tube. Just a thought...
Steve Baldwin can do it - Mike (the gunsmith at OPS) sends his barrels out to be cut down, I don't know where he sends them to, but you might check with him (he did a contender barrel for me that ended up pretty nice).
I think a good smith can cut the tube mag, cut a few coils off the spring to the right point, and re-attach the old 'lid' with a detent or the lid itself for a spring stop, however it may be set up. It can definitely be done; just a matter of finding a qualified gunsmith to do it - or do it yourself. Then, cut n crown for the barrel, re-blue, and possibly re-attach the front sight (if not an optics-only gun).
I think it's a fantastic idea - especially in .35 Rem. The Maurader 336 is the old discontinued 16" in .30-30 - too bad they don't bring this back. Dunno if they ever made the Marauder in .35 Rem. The Winchester Ranger was the 94 in 16" .30-30 - these can be found for sale used much easier than the Marauder. Doubt that any of them were in .35 Rem though.