71 Warbird, they will cost HP as compared to good straight through mufflers but compared to chambered mufflers, it will be basically a wash when they are open. But, you are already giving up some HP for the manifolds as compared to headers anyway so I would not get too overly concerned.
They will be really loud on that engine when opened and should be very quiet when closed..the big 455s really crackle when open.
Tom