What is the port size difference between 318 and 340-360 heads

Having a smaller intake manifold port than the cylinder head port is usually better. A bigger intake size can create a minor "dam" effect and cause turbulence, which may negate some or all of the advantage of a bigger intake manifold. So the "small" port Edelbrock Performer fits & works fine on 318, 340 or 360 heads. The Performer RPM has bigger ports and would be more "productive" than a regulat Performer on the 340/360 heads.

Gaskets are usually a hair bigger than necessary to avoid sticking out into the airflow.

I don't have the actual sizes, but there may be very minor differences, as well as core shift.