Is this for your auto-x car? You have something else to drive every day, right? If so, remove the heater core, AC coil, bloser fan, and ventalation system entirely. If my memory serves me correctly, the AC stuff altogether doesn't really weigh that much, but if you've already upgraded your radiator, you can do without the heater core's liquid volume and the rest of what's behind the dash. Just run a hose from the inlet to the outlet where the heater hoses go on the motor, and lose all that liquid weight. No point in leaving the airbags in there, either.

If you intend to keep the car street legal, I really wouldn't suggest going that far, because every street driven car really has to have heat. You have to be able to defrost your windows. But I THINK there's no law on the streets saying that your car has to have working air vents... At least not where I live. Might be against some rules for racing classes, though.