Dave Edward's site lists the gasket on this page:
http://www.autotran.us/hmparts1.html
*Fluid Coupling Cover (Torus) O'Ring (rectangular x-section) for 1954-62 Chevrolet PU, 1952-57 GMC
6 Cyl. Models 100-340, Also fits some Nashes (1953-56 E & Y) and Hudsons (1952-56 Z) HG-10
3333
8614291
$ 10.00
I don't know if he actually has it.