I have an '06 Silverado EXT cab 6.5' bed, the front of a stock Foreman will press against the cab's rear window if you close the tailgate. Just as joecamel suggested, a Louisiana Guard Dog with the lockset is just the trick. It'll leave you with about a foot of clearance in the front of the bed, your ATV won't be moving or shifting around at all, and with a little practice it only takes about as long to hook it up as it would using straps to secure your ATV.
As long as you've got a 2" receiver hitch on your truck and a 2" ball mounted on the back of the Foreman - I have a Reese setup with 3/4" shaft and interchangable 1-7/8" & 2" balls - you'd be good to go.