I've rebarreled at least a dozen Marlin X guns with A&B and Savage take-off barrels. Since I have a lathe and finish reamers, I always trimmed the Savage specs barrels back 0.05" and recut the chamber to insure correct head-space. The Marlin bolt faces have a slightly deeper recess than the Savage bolts so you can get a false head-space reading if the front of the Marlin bolt touches the rear of the Savage specs barrel before the head-space gauge has touched the front of the chamber.
I know that a lot of people screw Savage-specs barrels into a Marlin X action, head-space it by feel, maybe with a layer or two of packing tape on the case head of a factory round, and call it good.
I've done X guns in 22-250, 250-3000, 257 Roberts, 6.5 Creedmoor, 260, and 7x57.
