C5 is 1965, C6 is 1966, it will work fine. Someone has milled the head to gain compression, either lost due to not being able to find the original head gasket which is a lot thinner, .027 and hard to find, or simply to increase compression ratio. Replacing with a newer, thicker gasket, new head will have to be milled to make up the difference in thickness or lose compression. Stock combustion chamber measures 51.5cc for 1965 200ci engine.
66 Coupe 200ci I6 Stock
66 Bronco half cab 203ci I6, CI aluminum Head and intake, Holley 390cfm 4V, Clifford header, Isky cam, KB pistons 9.7:1 comp, DSII ignition