Power loss. 318 Vs 340 manifolds

I have a set of Magnum truck manifolds I bought to use on my 73 Dart and there is no way they will clear power steering. Also the outlet on the driver side and somewhat on the pass side is right up next to the firewall. Head pipe would probably be in way of shift linkage also. The bigger the cam the more the HP manifolds would gain the cam used in 300 HP crate motors is pretty small so real world numbers will be bigger then in the article. It's going to depend on if you want max power or not, then IMO headers are the way to go. But if your only planning on 250-300 hp with a mild 318 build I still think manifolds might be the ticket.