Pics of your Roll cage main hoop mounting

I would just butt the plate to that flat vertical edge of that higher 'shelf' area and keep it on the bottom floor level. Keep in mind the tube doesn't have to be welded in the middle of the 6x6 plate.

On my 66, and like a ton of people on this site, I welded the main hoop to an outrigger going from the subframe connector to the rocker.