I'm really starting to like the FG Jeep body after looking at more pics:
The FG truck has a wheelbase of 475mm (18.7in):
FG Monster Jeep
Looking at this pic, the Jeep Body seems to have a slightly shorter wheelbase than the truck itself:
So I'm thinking that I could build my truck an inch shorter - 17.5 in wheelbase - and have it fit nicely under that body. But I'm still open to other ideas.