I'm not sure if the question is directed this way, but I wanted a new set of keys to replace my worn out GM originals. And I didn't want the "Fits GM cars" type that locksmiths and hardware stores carry - I wanted a set of real GM blanks. All the major vendors carry them, I believe. There are different letters stamped on the shank for both the square and oval keys that correspond to certain years. According to Corvette Central's catalog, you would want a "J" square key and a "K" oval key as those ones fit years 70,74,78, and 82. Once you get the key blanks you can take them to a locksmith and they'll cut them to match existing keys, assuming you still have some. And if you don't have any keys, bring them the ignition lock cylinder so they can match the square key to it and another cylinder that they can match the oval key to - as Joel says.
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.