Its a good question Chris, I also wonder if i could not just hook up the green wire to the green wire on the coil and not use the GMR. There is not much published about this that i can find. On Corral and the MSD site and honestly most folks use an MSD box with the Tach/shiftlight port.
I may just wait and buy a box as a new GMR is over $90 to my door, a good 1/3 of a 6al box, especially considering this one is already kaput and is new.