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. Something that can't be measured on a dyno is an engines rate of acceleration, when strokes get very long, engines don't gain rpm as quickly as one with a shorter stroke. Though I don't really race, I've seen at the track where shorter stroke engines can get of the line quicker than some of these really big stroker. I feel the 408 is a nice balance with good heads.