i'd go with dual 2 1/4 inch pipe, a dual 2 1/4 inch exhaust is enough to support 450 horse power, you dont want to lose scavenging affect.
I know a guy who runs a 408 stroker magnum thats real close to 450 hp and he runs hedman hedders 1 5/8 primary hedder tube into a dual 2 1/4 inch exhaust with an H pipe and his car runs just fine with the exhaust set up he has.