On the few engines I have seen, the casting number is usualy about midway down the side on the left side of the case, an inch or so above the bottom edge of the block. They are raised letters that were cast into the block mold.
The numbers that are stamped into the case just behind the dizzy is the serial number.

Looked thru the lists for GMC head casting #'s and didn't find the one you have. Might be from a Chev engine?... All the early GMC #'s I found started with 2.

Either way, locate the block casting # & you'll know for certain. You can bounce it off the list at http://oldgmtrucks.com/GMCCastingNumbers/ & see what year range & size the motor is.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P