Jeff,
I'm not 100% sure right now, but I'll tell you what I believe is the case here. As soon as I find out the answer, I will write another post.
Right now, I believe that " YA" is the only correct code for a '70 automatic 455 GTO. Now, I know to "never say never" in this hobby. I'm sure that there have been engine mix ups at the plants, as well as undocumented running changes that others may be able to share with us. Again, this is just my gut instinct in this matter.
I believe that the "YC" code 455 for 1970 was the optional 370-horse, big-valve, engine available on the big Pontiacs over the "YH" code 360-horse, small-valve 455. Up until '70, different codes were used for the different model lines (A,B,G,F), even though the engines were essentially the same. In 1971, the codes were simplified, where the same YC-code 455 was used in Birds, Goats, Prixs, and Cats.
Again, I'll write another post when I find out for sure about the codes.
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