There shouldn't be ANY problem with replacing the current carb with the correct one. I'm sure they didn't do any tooling or other modification that whould hamper putting an original back on. As far as going to 4 Bbl, I don't know CA law. Here in LA, it's local tradition to replace a 2 Bbl with a 4 Bbl or 3 2Bbls as far as that goes. My guess is that the place you took it to doesn't
understand what they're doing. They know what they've been taught and nothing more. Try a couple other places, and I'm sure you'll find somebody that wants your $. Look for somebody with some
experience 
.