I've seen lots of spray foam insulated vans. Some pretty and professional, some not so much. Use a closed-cell foam and there wouldn't be too much to worry about.
The insulated van floors I've seen were all done on the inside with a second floor installed a few inches above the van floor. I did see one box truck insulated under the floor using rigid panels that were then covered with plywood.
If you can't get a recommendation here, ask the Canadians on Kleen Kuip.