a longer block is not needed. It could help, and I could see it happening in the future (like the NASCAR R07) but that is unlikely.

The old 400 block and the LS7 each had a 4.125" bore. The OLD 267 SBC had a 3.5" bore. THey all share the same bore center to bore center distance of 4.400 inches. It is possible to get a bore up to 4.200" but things get hairy around there.