OP here:
I have seen Chevy literature that shows these stiffeners attached (rivets, welded, or bolted - hard to say) more than once, so I think at least in some applications they were factory stock. The '46 I have was a dump truck for many years and then a farm truck. I've decided to do everything I can to pick and vacuum and force air clean between them and then force POR 15 into the cavity and cross my fingers.
As a wise guy once alluded to: Life is too short . . . . Thanks for all input Mark
