The boxlock. It fits. Nobody who bends stocks can or will guarantee it wont break when he does it, or that it wont move back. You dont mention if either is an ejector (more potential trouble, but, I digress) but, keep in mind a sidelock NE is damn near impossible to sell. It sounds like that one has been used, a lot.
If it is unused, the boxlock likely needs a service.

Good luck.

Best,
Ted