If you guys are still out there this 7+ years later, I'm curious how it turned out. I am in the same situation with a 56 31300, 700r4. Instead of cutting the rivets I am considering cutting the crossmember vertically along the frame to leave in the rest of the crossmember on either side since that is where the leaf springs attach. Then possibly shape a couple piece of sheet metal to weld over the cut cross section. Performance online says I don't need anything but this kit which might have not been available in 2015.
https://www.performanceonline.com/1...ransmission-crossmember-kit-with-mounts/I am dropping the front end 2" or so and hope none of this drags.
