How is a 273 different from a 318-340??

I have a 1969 dart with a 273 engine
It has a 4 barrel carb mechanical secondaries.
The secondaries don't seem to open.
Was the original manifold a 2 barrel?
With a edelbrock air gap for a 318-340 fit?
I will eventually replace the engine with a 340 and want to be able to reuse the manifold if possible.
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