The most successful conversion of the M1 Garand to a box magazine was done by Beretta. The BM-59 was a magazine fed .308 caliber rifle used by Italy.
About 15 years ago, Springfield Armory imported them and conversion kits to adapt existing Garands to the BM-59 configuration.
Similiarly, John Garand modified the M1 to take the BAR mag too. He did this after examining some field expedient models which came home from the front. The only trouble was the desire to make the gun full auto. The BAR magazine did not feed reliably (bolt was too fast) and Garand suggested either modifying the BAR magazine (thereby making the two guns incompatible) or lengthening the receiver. He choose the latter.
Remington also studied the project and modified the recoil spring guide such that it impeded the rearward travel of the op-rod, thereby allow the existing BAR magazine to be used.
Getting back to the 308 caliber box mag fed Garand, not too long ago somebody was converting them to take the M14 mag. Use to see the ads in the Shotgun News. Sorry but I don't have the name or #. As for myself, I'd like to have one which accepts a BAR (30-06) magazine (in semi auto only of course - can't control a Garand if full anyway).