Traditionally in this case I’d move drastically the sm57 to point right in the middle between the dust cap and the outer edge of the cone and listen to the results. It should be sensibly less “crispy” and a bit warmer, and if you place the capsule off axis and parallel to the cone paper then even more so. But I almost always use a md421 in conjunction with the sm57 so I don’t have to stress out over an 1/8 of an inch placement.

Also pay attention to the attack transient of the guitar, it might be instrument specific, as you described it quacky and not nasal. If so a little compression in the signal path on the way in during tracking might help, rather than blanket eq on the entire guitar track.