Better gas mileage?

I just bought a 2000 z24. I like the car very much but I figured it would get better mpg's than 27. It runs good. The trans slips a little tho. Befor I got the car it sat for a long time it's pretty rusted underneath , I'm thinking that the muffler and cat might b clogged a little. I'm going to put a new muffler on it. I was thinking of a flow master. And new cat converter. Also I'm thinking of getting a cold air intake. Has any one tried the cheap ones on eBay? Lol. Would it b possible for me to get 35mpg out of my car? What would I have to do?