This reminds me of a totally unrelated story.
Back in "the day", our little computer company would rent minivans to load our computers and stuff in to take to NYC for Unix Expo (nevermind the fact that the rental agreement clearly said we wouldn't drive them in NYC or Mexico...at least we half complied). The second year we did that, we noticed when we went outside to take the vans back to the office from the rental place that all the seats were cable locked into the van (and I don't just mean locked together, I mean locked to the floor). We went back in and asked if they could remove those locks and they said no. We said they could keep the seats *and* the locks as we just needed empty vans. They still said no.
My boss just said "okay, no problem...we'll just leave them in." When we went back outside I said "what are we gonna do? we can't fit everything with those seats in there." He just said "I'll meet you back at the office." He showed up back at the office about ten minutes after me and gets out with a new set of bolt cutters and padlocks. We cut the locks, removed seats, and used them like we needed to.
When we got back and put the seats back and locked them I said "you gonna give them the keys?" He replied "hell no...they probably lost the keys to the ones they had on there anyway." And he promptly chucked the keys in the trash and we returned the vans. They never said a thing.
Back on topic, I can't see modding the truck to tie a car down. It'll be difficult to get a sturdy enough tie-down. Add to that you'd need some mega-ramps to get a car into the back of a truck like that anyway. Two car flatbeds do exist...see if you can find one of those to rent somewhere first. Probably won't have them at U-haul, but some other places might.
--Donnie