Jeff,
Boy was I wrong on my first answer to your post. Please disregard it completely!
"YC" IS THE CORRECT CODE FOR YOUR '70 AUTOMATIC GTO! After digging out the '70 service manual, and Pete McCarthy's bible, both clearly show that YC is what you need. The GTOAA Legend has it listed incorrectly as YA, which is what I referred to in my first response. I made a note in that issue.
The 370-horse, big-valve 455 available in GPs and big cars is code "XF"!
Sorry about any confusion that I may have caused Jeff, and to the general readership.
Since even I can blunder when it comes to engine codes, I suggest to everyone to get an original service manual for your car. They have these codes clearly explained at the beginning of the engine section. Another good source is Pete McCarthy's "Pontiac Musclecar Performance 1955-1979". In it, he has all years listed.
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