I don't think it is loosing the bed length but the height of the box. Most 5ers only have around 6 to 7" of clearance between the bed rails and the bottom of the trailer over hang. So you have taken over 2' of bed length away from the bed and when you do a turn the trailer is going to turn and try to occupy the same space as the tool box. Figure the hitch places the pin about 2" in front of the rear axle so as you trailer starts to turn at up to a 90* if backing up. Not good situation.
I have a Mega cab Ram which only has a 6'4" bed but I'm running a sidewinder that moves my pivot point on my 5er back 22". So this gives me about the same distance from the cab to the pivot point of the trailer as a long bed. My 5er has about 9" from bed rails to the over hang of the 5er. When I just turn the truck going forward I have about 12 to 14" between the 5er and my cab.
Now even if you had 8 to 9" of bed rail clearance and was doing a hard left or right turn and the trailer wheels on that side dropped into a hole that .5 to 1.5" clearance woukd get disappear very quickly. Also if the truck tires could fall into a hle that you didn't see and this would be the same results.
In all of this I'm saying not a good idea IMO.
