Recently, a lot of people have been recording guitars with one dynamic and one ribbon. Usually a SM57 and a Royer R-121. I have found great success with (2) SM7b mics.
What I do is take the grill off one of them to expose the metal and the capsule (yes the foam grill on your SM7b pops right off and can easily be put back on). I position that mic in the center of the 12" speaker approx. 1-2" back. Then I take another SM7b and seat it to the right of the foam-less mic so it is position on the edge of the same speaker cone at the same (1-2") distance. The center SM7b is brighter since it no longer has the foam and the other one is darker. The SM7b is a thick sounding mic that can take a lot of SPL, so it is easy to eq and sounds great! I usually record with Neve 1073, or 1081 micpre/eqs and no compression. For thick heavy guitars, I usually prefer this mic setup over the R-121 and 57 setup. Taking the grill off the SM7b opens up a new world and makes them sound like (2) different microphones that complement each other very well. I hope you get to try it some time!!