"If you can't raise the bridge, lower the river."
This old saying would surely apply here.
Since the gap between the cab and bed is too narrow or restrictive, simply INCREASE the gap! I'm thinking that 6-8 inches would work, but 2 feet would sure be luxurious!
By removing just a few bolts, the truck's bed will "easily" slide aft upon the chassis rails and the wooden spacers between it and the bed.
the motive force necessary to make the bed slide can be a chain and a tree, or just a good ol' 2K lb. Come-Along. Of course, you want to be sure that the force is applied STRAIGHT with the Bed Frame and the Truck Chassis so they stay stacked properly.
To realign the bed to its original position, simply reverse the Come-Along's direction of pull, OR gently back the whole truck against that tree and carefully slide the bed back until the bolt holes all line up again.
When we first acquired our Deuce, it came as a Cab & Chassis and we acquired the bed the same day we took possession of the truck. A small forklift set the bed onto the chassis, then in less than half-an-hour using a Come-Along and a Pry-Bar, the whole alignment and bolting was a quickie! I simply chose a place on the chassis and another on the bed's under-structure for using the Come-Along. When the bed tried to slip a bit to either side, the Pry-Bar readily "steered" it back onto the truck's Chassis Rails.
I hope that explains it well enough to help you.
Give it a shot.
