Engine Coolant residue through vents

I have a 1970 Impala and was wondering if anyone knew of any possiblities to the reason why I have coolent in steam form coming through my vents when I accelerate. It comes out like the engine is on fire when I accelerate but if I just let it run in my driveway forever it barely comes out.

Sounds to me like you're heater core is shot. When you accelerate, the water pump spins fasterand puts more pressure into the system. The only way coolant can get inside the car is through the heater core. Replace that and evrything should be fine.