WIW All Original 74 Duster 360 4 Speed Car?

The best factory time I could find for a 360 duster with auto/3.21's was 15.80's. This seems slow to me means I've had factory 318 2bbls with auto/2.94's go 15.90's. I do know if they could only get 15.80's and was around 86 mph, something serious was holding that car back. I'm not talk'n motor internals either. I ran 15.30's with a D150 100% factory engine (360 tbi) including single exhaust/stock manifolds and 3.21 gears.