Are not the molds still around from the original 340 blocks? My understanding, all the 340 blocks were from the same casting molds. 68'- 73'. Where are they? Finding them would cut the cost down from starting from scratch. Maybe offer the "TA" (4 bolt main) instead of the light weight 340.
I googled this to be sure, no factory 340 blocks came with 4 bolt mains. The block did have thicker webbing if you wanted to go to 4 bolt mains. The aftermarket blocks are a 4 bolt main bearing block.