I have had 3 64 Plymouths with the 426S, and every one of them was a slug. Well, maybe not slugs, but a strong 383 would walk them, and I have had more than a few 64 383s too.
Problem was the 426S was a mild motor, used the 413 New Yorker cam, had no better heads than the 383, etc.
With better cam, heads and intake would have been a different story.
Still, a factory 426 car would be a damn good starting point, more for the other stuff that came with the engine package. Think bigger (11") brakes, cop car(as good as it was) suspension (with front sway bar), HD cooling, and glam factor.
Wish my 65 Coronet convertible was a 426S pkg...