hey guys - I put a scan of the seal from the assembly manual up
here [click "
full size" upper right] - the piece will be the same panel/burb 55-59 Chev or GMC ... it has been repro'd cuz I've got one in the parts stash for my project, from Golden State Pickup Parts, part #17-6RPS under bumpers [no picture provided]
it is/can be just a straight piece of rubber about 3-4" wide if I remember right, just wide enuff to curl over and 'wedge' behind the bumper - the metal piece that holds it on the valence is channel shaped in the manual, but the one on my panel was just a flat piece and looks original - just held with screws into J-nuts
I'll dig out the rubber sometime and get measurements, and post here .... sometime

Bill