Hi Gary
I have found these as NOS or reproduction parts in the past. Check out some of the Chevy Antique parts sources. As suggested you may even find it through your local auto parts store just say its for a 1938-51 Chev truck. Quick check of the civilian master parts book that period shows lots of different sender units but interestingly only one gasket which would seem to say the screw pattern is the same. Suspect that the only difference is in the length of the arm. Places like LMC
http://www.lmctruck.com/icatalog/ca/full.aspx?Page=83 sell a universal style slightly different from that used in the CMP but I think they fit the same attachment screws.
Please let us know if you find a source.
Cheers Phil