If I read this right, you are looking to put a toad on the truck in the gap? Must be a small 4 wheeler. Not to mention the handling issues. A golf cart might work better.
Never thought about a 4 wheeler up there so my thoughts might be way off base on handling and loading. I went with a toy hauler trailer to cover the 4 wheeler hauling issue.
BIKES fit great in that spot. There are several different setups out there to get the bike on the truck. Full size baggers (Wings, Kings, etc) fit without issue. I've put a flat platform on the back of Suzy Q and have had a VTX and Valkyrie on the platform at the same time. Didn't try it with the trailer hooked up, but it looked like it would be ok if U-Turns weren't in the mix.
One example of a bike hauler is at
www.douglasstruckbodies.com/pages/index.htm. (On the menu, hit Recreational / 5th Wheel and scroll down. You'll see it).
I also saw a unit on the web that had a Saturn in the space behind a day cab. The car was nose-up and had been modified to prevent problems. Radiator, battery and gas had been tweaked. Can't find the site now, will keep looking for it.
Curious what you were thinking of hauling? I'm interested in the engineering process.