I know when i removed my 67 seat raisers, they were spot welded in front and back as well as a tab on the incline of the raiser, I simple drilled these out, but i also ground some welds on the top side of the pan on each side to get it to remove without a hassle.
If i were you i would try and find a spot weld remover bit for the job, so you can try and only cut through one layer of metal leaving it easier to weld the raiser back in. I have never used one, but heard they work very well and make rewelding much easier. Anyone used one of those?