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There are documented 70 L78, and LS6 chevelles including the convert you mention. All with 842 heads. Your memory is fine.
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Whoa, whoa, whoa...not doubting you a bit, but did I sleep through this somewhere? Are you saying there are *legit* '70 LS6 Chevelles out there with aluminum heads? I thought the alum heads were a '71 LS6 'Vette thing? Sounds like a new setup for Pure Stock!
And the mention of the 842 heads is important...this isn't always as simple as a head exhange...the pistons/heads have to match (open/closed chamber), and the rest of the crank/rod assembly has to match the piston (counter weight on crank, pin configuration)...which means diggin' into the engine.