1971 was a weird year for the Firebird. You could get the base 325 HP, 455 4BBL with the regular cylinder heads in the Formula and also the 335 HP, 455HO with the round port heads, for a little more. They didn't sell many of the 325 HP version. Rare but not that valuable.
In 72 it was just the 455HO version of the 455.
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