I'd look for a longer mag tube first, welding 2 tubes would be a last resort for me. Marlin Owners is the place to go for this kind of info, use the search tab for "extended magazine" etc. I'm working on a project gun that started with a 336XLR in .35 Remington. I'm having the barrel cut to 18.5" and a new dovetail cut for a longer mag tube which I ordered from Numrich along with a new spring. The gunsmith said 2-3 week turnaround so I should have it all put together by the end of the month.