Hey guys, thanks for the post. The box/van body I bought was made to mount to a truck frame already so I was able to mount the box the same way they mount them on medium duty trucks with u-bolts. I put down 3/4" thick oak between the frame of the truck and the frame of the box and then u-bolted them together every 4 feet. The boxes or van bodies usually have "I" or "C" frame rails that run front to back under them. Depending on if your reefer unit has those you would probably want to add them. I think most trailers only have left/right I beams every 12 inches or so? If the rails weren't going to line up on my box/van body I was going to add my own "I" or "C" beams to the box/van body where they needed to be. They are just welded to the left/right beams. Wouldn't of been that difficult.
As far as the boot that goes between the box and the truck cab I haven't bought that yet but have sourced it from:
http://www.accordionboot.com/
We put one of these on my father in laws toter and the only advice that we learned is buy the extra wide one. It is more money but we bought the regular one and couldn't get the boot on the right way because it wasn't wide enough. Really need the box to be about an 1" no more than 2" away from the cab to make work.
Take care,
Steve