273 intake on a 318

there's simply no way what you describe can have anything to do with the fact that you have a 273 manifold on a 318.
even the intakes with the different bolt angle have the exact same port runner size and spacing.
I have 273 heads on my 318 with a 318 manifold. I only went with the 273 heads because they are closed chambered. if you like I can go out and take photos of a 273 head next to one from a 318.
your issue has to be something else.