The old 6.1 style aluminum Gen 3 blocks that was designed and cast by Arrow Racing are sold out from PREFIX. We have one of the last blocks as the 73 Challenger build's 426 engine. There may be some available at Summit or Jegs?

Mopar has a 6.4 aluminum block P5160138 which is cast by TEI that we saw at SEMA 2018 and also there is Gen 3 Perfomance H3MI 357 aluminum alloy block, maybe a few billet options out there that are available but at much higher price.