Remington has made 40X repeaters, for use in the US National Match program.
I've not done it, and I think that if I had a 40X single shot, I wouldn't alter it, but there would seem to be a couple of options.
One would be to mill the bottom of the receiver to duplicate the M700/40X Repeater receiver. This would be more complicated than it appears at first glance.
A second way would be to mill a simple slot, which would clear one of the center feed aftermarket detachable magazine systems, like the PT&G.