Decent Slant 6 carb, intake, header setup

I played the Slant 6 game for 10 years. I even had Doug Dutra build my motor with most of his speed secrets for them. It was a great running Slant 6, but it was still a 18+ second quarter mile car.
I have to ask why that slow in the quarter, I did better w/a mild Crane cam, 3.55's, 318 BBD, 6-1 DC header, stock valves........17.0's @78mph.......
what's up with that?