I have a similar 451 with 906's, performer, and stock big block a body exhaust manifolds. It makes right at 300hp to the wheels on a chassis dyno as it is driven off the street, heat soaked and all. Uncorking the 2.25" repro exhaust is work 20rwhp.

The 451 in my dart made a hair under 500hp/500tq on a realistic engine dyno with a Performer RPM and 1-7/8" dyno headers. I figured the manifolds -50hp, intake -15, full exhaust -25hp, factory magnum air cleaner -10hp and don't forget the accessory drives for a very believable 100hp loss at the flywheel. That leaves us with 400hp going into the clutch.

400x.80=320rwhp on a perfect day. Take a car driven in off the street (heat soaked) on an 85 deg. day and 300ish at the wheels isn't out of the ball park.