318 MAX fuel economy builds?

It’s about equal. Build dependent.

Factory dual plane, Edelbrock Performer & the SP2P for mileage builds.

Any dual plane except the SP2P or the Offenhauser dual port.

The thing here on a mileage build is beating the factory intake in the low rpm range. The SP2P intake would be a disaster for a performance intake. Though it was never built for performance. It was introduced during the gas crunch days.

I had a coworker tell me it was a good help in his van. It is possible but more likely improbable to find a two barrel SP2P.
Crazy hard to find they are.
The one below is a picture gleamed from the web.
AKA - Not mine.

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The below standard Chrysler 4bbl Thermoquad intake is all you really need for a mild performance build or a mileage master searching intake. For its design, it’s hard to beat.

View attachment 1716014564
I have one of those somewhere in my parts stash. I have a TQ somewhere also.