While I'm not that far on my Sub, well I'm not even close! I have built some conversion vans back in the day. We always used at least a 4" diameter 1/8 to 1/4" thick steel washer for mounting under the floor pans. Since the Subs have a plywood floor in the back, I'd run a fairly stiff channel tied from side to side on the body then bolt through that. we never expect to have a need for such safety, but then you never know. Better to be safe than sorry. I'm hoping to use a mini van floor (cut out from a handicap conversion) in back of mine, preliminary measurements say it will work. That will give me a solid floor, plus mounts for seats and seat belts on a steel floor. I've got so many repairs to do to mine it will never be close to stock, so I'm doing some things differently.
Good luck!
Inuke
'49 Fleetline, '49 Sub
