Nothing wrong with a 6-71 on a 454.

Our 21' ski boat has a 4 bolt Gen IV 454, 7.0:1 CR, rectangular port iron heads and what would be a fairy radical cam in a NA motor. We run a Delrin ribbed Littlefield 6-71 on it under an Enderle Bug Catcher with an EFI spacer. It idles like a little old ladys' Buick, starts within 1 turn of the crank, and I never have to wrench on it. The pullies are set to 5% under which gives us around 7-8lbs of manifold boost.

If I wanted more than 700hp I might want to upgrade to an 8-71 to avoid saturating the blower, but I'm happy where it's at right now. A Roots blower is nearly magical, I can't imagine building such trouble free, well behaved horsepower any other way.

I would recommend using a blower set up by a known shop, not too sure about those Weiand things that Summit sells.

Regards, Mark