AmmitOsiris - if you are building a rig that will not be registered for the street, then you can put whatever you want on it. If you are building a rig that will be driven on the street then you probably should stick to what is legal. From what I understand it is a violation of the California Vehicle Code to use anything other than stock or C.A.R.B. approved parts on a pollution controlled vehicle in California. From what I've heard, CHP or local law enforcement can ticket you for not complying with the law regarding this matter Maybe timatoe knows if LEOs actually do this, I've just heard rumors.
Its kinda lame that a weber carb can pass emissions for the 1990 4.2L (in your case), but it fails visual because it isn't approved for use on the engine you have. Thems the laws we get to deal with in Cali though. Not sure about Brentwood area smog restrictions, but you'd be doing yourself a favor by adhering to smog laws for street vehicles if you are planning on keeping it street legal.