Is there a list somewhere stating what states have emissions laws, require visual inspections, or just sniffer tests?
And if you fuel inject an old car, does it still go by the old engine's emissions requirements or some new set?
Are there any odd loopholes that some states have also?

 

All in one place? Not that I am aware of. Is there something in particular you are looking for?

Generally, in most states, if you change just induction, you have to pass emissions of the year of your car, not the induction.

 

A more general question would be "could I buy a 75 Z/28 with a 400 sbc that has a 4 barrel carb, and then replace it with a fuel injection, would I have to pass smog laws for the old 400 or the new EFI'd motor specs? It is a hypothetical one.

 

It should be for the year of the car.

But it comes down to the state. In NY State if the car is 25 year or older there is just a safety inspection. (this is for western and upstate NY, down state near the "city" the rules may be different)

 

Try this web site.

http://www.epa.state.il.us/air/vim/

Your best bet is to ask someone in your state about that, I don't see anything specific on modifications.
Emissions testing is very regional. Here in NC rules are different by county. My county only requires a visual emissions check and safety inspection for my '87. No sniffer or dyno, they just look to see if the cat and egr valve, vapor canister, pcv valve are there.