Yes to fit cage, or fitting lower but further back seat. basically i have no use for rear seat bin. Can i weld a sheet of thick metal on pillar and the floor. and mount the seat thingy that pull the seat belt in on it?
I think I would be more inclined to cut the bin, leave the metal around the belt point and reinforce it with more metal of the same size rather than cut and reweld a bit in.
Have to retain the bins in my rally cars to meet the regs and I also have cages in both. It's a bit difficult to drill off for the 'B-Post' cage legs with them inside the bins, but not impossible. The difficult bit is holding the bolt heads to tighten them up really tight.
I think one trick is to get a long punch (or extend a short punch) and with a goodly whack mark the hole centres from the inside, then drill through from the outside. I havent tried this, i removed the bins and fitted harnesses.