I think you should stick with the stock airbox for the ti. I don't think there's any benefit to a 328 one. The ti filter has more area so it should allow more air in. You may actually decrease airflow with a 328 airbox. That seems counterintuitive to me, but the filter is smaller. The boxes are very different from each other, so I'm not really sure how much air each one flows. I can't tell from looking at the parts diagrams if the 318 MAF will fit in the opening of the 328 airbox. Check out
www.realoem.com and look at the diagrams. Maybe another member in CA could let you borrow his airbox for your smog test.