I personally feel your best bet is to build a ramp raising the front of the ATV high enough to close your tailgate. After pulling a light TT similar to yours with a F150 and a 650 Kawasaki ATV in the back for 1000's of miles, it can be done. Will you be at your limits or a little over? Probably, but it is very doable. The only difference between what you are talking about and my previous setup is that I had a 6.5' box and was able to strap up the tailgate with ATV sitting flat.
Yes, as many have said, a TH would be better but what you are talking about can be safely done without problems IMHO.