2bbl to offy dual port questions....

Since you're not doing anything to raise compression or open the valves longer, the 550 seems a good choice. If the car has an automatic, I recommend a vacuum/air valve secondary carburetor.

Hot Rod did a test of 18 manifolds on a 383. The Offy dual port produced 40 less hp than a Performer RPM (412 vs 451). IMO, the Edelbrock Performer will give better performance and will match up with the ports on the 318 cylinder head.