[slowmover]

A 5'er tends to be quite wide. That's a wall pushing aside a lot of air. In a head-on view these things can just dwarf the truck (big fish pursuing little fish, mouth wide open).

There have been some custom hauler beds which appear to take advantage of the "gap" for storage and best clearances. But the cost is prohibitive and the simple utility of the pickup is lost.

Aerohead's archaeology diggings brought up a trailer nose from 1982: fabric, inflatable (Fibrelock; search Pop Sci 1982). Something like that could both fill the gap and ideally not damage either vehicle if it had a little give (for the rare times interference occurred with an otherwise optimized design).