71' 340 air cleaner, choking out my little 318. WHY??

You can always play with the jetting & air door. But there are a couple of things, or combination of things, that could be causing the problem.

I think the air cleaner top is too close to the carb vents, causing turbulence and/or flow restriction.

Can you temporarily space the lid higher without the air cleaner? If that works, you may need a taller air cleaner.