I think that Denny's link is to The Filling Station (their catalog is not easy to link to). They have a dark grey (blue grey?) that many people think is a good match (as Denny posted) at least for pre-54 truck engines.

Engine Enamel Paint - Dark Grey
FS-359 (aerosol)
FS-119P (pint)
FS-119 (quart)