Parts for the 503 are just about nonexistent. I was able to find an upper engine set for a 54 GMC 503 at an obsolete parts vendor in Canada several years ago. It was the last one they had in stock, and it got used during replacement of a leaky head gasket. Making those exhaust gaskets might be your only option. Ed Pruss, a long term member here, owns that truck. I'd suggest sending him a private message to see if he can advise you n parts sources. Other similar GMC engines were made on the same basic design, including a 426 cubic inch version, and a couple of other sizes.
https://en.wikipedia.org/wiki/GMC_straight-6_engineJerry
