First off, I must say that this is a thing of beauty, and I salute you!
As far as feed... you could simply have it tube-fed from a spot in the back of the stock butt, similar to the actual '73 but central rather than at an angle. Pop the cap on the butt, pop the cap on the tube, apply one to the other to fill, cap, lock and load. A bit clumsy, but not insurmountable. Or you could go with a magazine forward of the lever like the Model 88, but that might impair its ability to be flip-cocked.