The climate was changing in the summer of '70 around the time of the press long-lead previews. GM was into being a good corporate citizen and dropped compression ratios in order to try and encourage use of low lead and no lead gas. It was up to the customer to actually buy the stuff. Meanwhile at Ford they had dismantled the Performance department. Chrysler still offered the Hemi for those who could afford to purchase/feed/insure the thing. Curiously it has recently come to light that Chrysler stopped casting the Hemi blocks in '69 but continued to install them into '71. Back to Chevrolet now. They were planning to put Z28 equipment onto the still new 400 mouse in order to cover the big-inch market & do away with the heavier 402 mark IV. There was a management change with a new set of priorities (can't blame them given the mood of the government) and all those projects were stopped; except for turbocharging (Twin-Turbo Novas) & wankel engine development (remember the new '75 Monza was to get the rotary engine but got the 262 as a stopgap). As for my opinion (just that) I doubt that Chevy actually made any LS6 '71 Chevelles, much less aluminum head versions (except for Corvettes). Even a Canadian '71 Monte Carlo owner's manual I have lists the LS6 as being available, but they just didn't come through.