I'm not sure any of them sell one pice pumpers but some might sell one piece bumpers. Scroll down the list and see who might come closest to selling chevy bumpers or could get them for you.
I did the scroll down but didn't see much so I thought I would just ask if anyone knew of any vendor.
Not an alliance vendor but http://www.chevsofthe40s.com/ has them. http://www.chevsofthe40s.com/detail/20002/Chevrolet_Bumper_One_Piece_Chrome_Front.html
I know Chevs of the 40's and NCA has them but was hoping to give some business to an Alliance Vendor.