You may already know this, but if you look at the frame from the back you have a problem there. The left rear rail is twisted and swayed, so both rails are likley bent.You would have to straighten the frame before you could put a bumper on, and probably wouldn't be able to put the bed on straight either.

1957 Chevrolet 1/2-ton Stepside LB in the Gallery
My Photobucket shots

The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." - John Stuart Mill (1806 - 1873)