340 intake on a 318?

With those gears, I don't think you will have a problem, but, no guarantees from me. I'm not a fan of mismatched parts.
It's not a mismatch. Chrysler built millions of 318s with four barrel intakes. To my knowledge, not one single one ever had small ports. I'll put it like this. I've never seen one and I've torn down a LOT of virgin engines in the junkyard. If Chrysler engineers didn't see an issue with it, then there was none.