If I'm not mistaken, with the additional stroke to make the 426 sb the rings are up into the wrist pin area, requiring support rails, possibly not ideal for a car that is driven. As
@replicaracer43 has stated, less than optimal rod angles, short piston, side loading, and limited cylinder head architecture. If you have ever been in a 408 without W2, W5, Indy, or seriously worked standard port LA heads, they nose over fairly quickly in the high 5, low 6000rpm range. If you add more cubic inches the rpm limitations become more pronounced.
