You can weld on tanks. Wash the hell out of it with a good detergent. Then trickle fill the tank with a constant feed of non-explosive gas (like Co2) while welding. This will remove oxygen from the equation.
I was considering a whole-new-tank instead of expanding the original is 'cause I can still expand it a little bit side-to side (maybe?), and construct it out of thicker "self armored" steel (and not need a skid plate). Problem with my design ideas is I've got 2 4-doors, and never seen the tank in a 2 door (which is going to be my next build). I didn't even know there was a frame width difference 'till this thread

If it's similar, the pinch weld is about 1/2" on each side, and there's another 1/2 or so between the pinch weld, and the frame (and front to back). It doesn't seem like much, but every little bit adds up when you're doing a custom tank. I'd have to look, but I might be able to get brave, cut a hole in the 'floor' and expand up into the rig somewhere too.
Maybe another super-crazy idea... but Buell's (motorcycle) actually have fuel in the frame (keeps COG lower)

Maybe that will spark some outside the box thinking...