18z, the following is an interesting idea. I haven't tried it myself yet so I can't attest to whether or not it will actually work.
A single stack magazine is pretty easy to design for the Garand without altering the receiver. Use a BAR magazine for 30-06 rifles and a M-14 or FAL magazine for the 308. Cut them in half, remove enough metal from each side to make it a single stack magazine. Use flat stockor a strip of light gauge sheet metal to join the 2 halves, by tack welding. The springs is a problem. The best I've found is from an 1891 Argentine Mauser, which is sold by Gun Parts. You have to use 2 springs attached to each other. You will have to rework the feed lips, and fabricate a catch/release, eliminate some parts and fabricate a few parts, but now you have the idea of how to convert the Garand to magazine fed, without modifying the receiver.
If you try it, let me know how it turns out. George