What motor do I have?

[Cowbound56332]

Hello, I just bought a 1963 Pintiac Catalina, and the owner claimed it was a 1966 Pontiac 421. I've been looking the motor over and apart from the YH code and only two freeze plugs, I can't make heads or tails of anything else. Any help would be appreciated. The date code area looks like "y78", front passenger side of the block just under the head on the left is "p275", right above the water pump is "312945" and "yh". Right next to the thermostat housing on the manifold is "A206".

 

[melsg5]

What is the block casting number and date code near the distributor hole back of block? The two freeze plugs narrow it down to 65 or 66 421 block.