You might need a good wire brush and some brake cleaner (cold engine) to reveal the casting #'s.
This is an incomplete list, but maybe your number is in here?
2135412 GMC 46-54 200, 270
2193980 GMC 52-54 302
2324002 GMC 55-62 248, 302
2324003 GMC 55-63 270
2324004 GMC 55-62 302
2404929 GMC 55-63 270
6107326 GMC 39-54 228
6107328 GMC 39-55 248
