From my 1965 Superior Interchange Light Truck manual on cylinder head interchanges:

#530377 GMC early S 8-336 (to eng #336-35001)
Pontiac '58 except 3 carbs with 2 barrels each and fuel injection.

#533777 GMC late S 8-336 (after eng #336-35001)
Pontiac '59 w/o 3 carbs with 2 barrels each.

Also not sure of differences. Hope this helps.

Last edited by randy luton; 01/13/2008 6:24 AM.
"Nothing astonishes men so much as common sense and plain dealing." - Ralph Waldo Emerson