A 74 won't go in for smog checks unless the legislature monkeys with the exemption, which they have done before. Generally they move it forward, not backward, though. It hasn't changed in about 10 years from the 75/76 split that is in effect now.
That said, cars are still supposed to have their original smog equipment intact. In theory there are roadside checks; if they sniff a high polluter they can stop you and look under the hood. I have heard of these, but never seen such a check. Plus, all the changes on a 74 are going to be internal, there's no external smog equipment visible, unless you change from Spica to carbs. So practically speaking, you don't have much, if anything, to worry about.
In urban areas, a car that visibly smokes can get reported to the local air quality board by a nosy do-gooder. I had it happen once with a Berlina, but it just generates a letter to get your car fixed or tuned.
Andrew