Last one I did I ground the heads off, removed the floor then with a sitting disc cut a slot in the bolt for my big flat head screw driver and with a bit of heat they came out, replaced with normal hex head bolts, M8 I think?
My MO on boot floors has evolved to this, new floor, new struts and support braces, if bad enough new side panels ( I have gotten owners to buy side panels just to cut them up to replace the floors seating flange), slice out old floor with old supports with a slitting disc, if not replacing floors seating flange then grinding disc on and grind spots welds off or buy a propper spot weld drill like
this(I found that grinding was faster than drilling, drill bit gets blunt fast when it hits a rust blister!), pre drill the floor for the supports, fit new supports, slot floor in and stitch an inch every 6 inches or so, seam seal and paint.
looks like you'll be replacing the rear crossmember too, just be methodical, rear door off and I welded a couple diagonal angle iron braces in the frame to stop it warping whilst I cut the rot out.