Short bed camper on long bed truck - Tips for frontal brace?

If you're concerned about lateral loading, then put a 3/4" plywood floor on the box. Glued and nailed to the 2x framework. It won't be going anywhere.

You could also build a similar box out of steel C-channel if using wood makes you nervous.

Remember that you've got fender wells, tiedowns, and presumably a rubber floor mat which all help to contain the camper. It's not like the camper is hanging like a pendulum and swings with its entire inertia against the front of the truck bed when you stop.