LS6 and LS7 were single 4 bbl and the LS7 was available as an over the counter engine assembly GM part number 3965774. This was a complete engine less intake and exhaust manifolds. It was listed as H.D. like the L88 and probably would have replaced the L88 on the option list if it had been released for production. The over the counter engine assembly used an iron open chamber head that was basically the same as the aluminum 2nd design ZL1/L88 head, the ZL1 camshaft and high compression ratio similar to the L88.
One odd thing I find in the parts book is the listing for a head for these 2 engines even though they weren't available. The older books have many listings for common options listed as "except H.D. ect. but few parts specifically for 454 H.D. (Some parts for special options that weren't released seemed to make it into the books on occasion. If you could have actually gotten them if ordered is another matter altogether.) It is part number 3981070 for both engines and noted as aluminum. From this I gather that if these engines had made it to the Corvette RPO list and went into production that the Corvette LS6 and LS7 would have had aluminum heads for the weight savings, where the production Chevelle LS6 and the over the counter LS7 would have had iron heads of nearly identicle configuration. The '71 LS6 had aluminum heads and seem to varify this point.
In response to the last post the LS6 would have been the big block counterpart to the LT1. Both carrying the nomenclature of Special High Performance and both being 11:1 compression, solid lifter cam engines. The LS7 was the 454 equilivalent of the L88 and both were labeled H.D. (heavy duty).