Towbar/Dolley for moving car around

I just made a simple cart kinda like what you're describing. Using it for a Ram Quad Cab. Realize the trk cab isn't as heavy a car shell but I'm sure you could do something similar, but build it more heavy duty to hold the body.
It's basically a 'table' w/ 8" casters @ the corners. A couple holes in the top to run two bolts into the front body mnts to keep it on the cart, when moving it around. Maybe you might have to make some kind of fixtures for your mnt points to secure the body to the cart and use larger casters.