My pump gas 408 ran 6.57 @ 108 in my 3260# 69 Dart. After running it for five years I pulled it to freshen it up. What I found was odd main and cam bearing wear from the front to the back. I've talked to and had several guys that build engines look at it and they came to the same conclusion that I did, block flex.
This is a 71 block, align hone, .030" over, square decked and balance rotating assembly. I don't know how much HP it made but I'm satisfied that 50 more would've busted it.
Good luck.
