A good ground strap to the frame is never a bad idea. But,
originally from the factory the box would be grounded at
numerous points thru the fender to the running boards and
aprons via their bolts to the frame. The current draw is very
small for the single or dual tail/brake lights of the day so
it didn't require much of a path to complete the circuit.

If the restoration shop was SUPER careful when assembling the
truck it is possible, that it was kept insulated by the layers
of paint, primers, gaskets, fender welting from establishing
a good ground. Tightening the bolts SHOULD have cut into the
bare metal enough to establish a ground but it is also
possible that it didn't.
Bottom line is.....ground straps are a guarantee that
there is a complete ground return.
dg

Denny G
Sandwich, IL