Vetterli 6.5 Carcano

[vintage hunter]

Before you go to cutting at it take a little time to find out just how it was originally attached. Chances are good that it'll either be a hidden non adjustable dovetail or it's a saddle type soldered straight to the barrel. Either way a little heat will reveal which one it is. I understand your concerns about reattaching it to the good barrel with the soldered sleeve but it is possible.
JB Weld and most other hardware store epoxies are way over rated for joining metals. The expansion/contraction properties aren't even close and the slightest bump will break it right back off.
My suggestion is, once it's determined just how it was initially secured is to use a low temp soft solder and a heat sink of some sort to reattach it. If it does turn out to be dovetailed it's possible it could be staked in place good enough and no heat would be needed at all.
Any good radiator shop worth their salt will know just how to fix it if it needs soldering on.