I was wondering if there are any mods I can do to improve performance on my 400 cid that will pass smog. Carb or exhaust?

 

I looked at Doug Thorley and Hooker and can't find smog-legal headers, so you are probably stuck with the manifolds. You can probably increase the size of the single exhaust.

You have a cat?

Have you had if for a while, and smogged it before? If so, post the numbers and the limits.

My theory is if you have a high limit on Nox vs. what your engine produces, you might be able to at least run the cam straight up (instead of retarded) or maybe go as far as increased compression.

That depends on the limits allowed. '78 specs should be pretty high.