YX is most likely a 400, though it was a 455 in 1974. The block casting number is behind the passenger head. We need that, too.
Also, the head casting code is on the center two exhaust ports. The heads are either 1968 or 1978. We can tell from that two letter/number code what they are.
What year car was this out of? The casting date J017 translates to October 1, 1967, if it's an early car or it could possibly be October 1, 1977. There is probably a D and N with an arrow pointing either left or right on the heads. If the arrow points at D it was cast during the day, N for the night shift. You'll need to look on the center exhaust ports for the head numbers. 67's used cast numbers 140, 142, 143, 061, 670, 97 & 997. If it's a 77 400 it probably would have 6X heads. I have a YX block and it's a 71 400 that came out of a Firebird with a 2 bbl automatic. You'll know from the date code next to the distributor hole.
