Are they still on the car? An exciting way to do it is to rev the car up, and pour 1/4 cup of water about every 20 seconds down the carb. Just don't have anything behind the exhaust pipe, because a world of crud it coming out.
If the heads are still on the car and the engine is in running condition, the water down the carb bit works well. Just be careful not to dump too much down too fast, but as long as you are careful, you'll be fine.
And my method of removing the carbon from the combustion chambers, valves, and piston tops.... a scraper and a wire brush. It was a pretty thick layer of carbon on some spots, 1/8" thick, and it was packed tight! The scraper followed by the wire brush/wheel worked for me. Have fun!
I never said it was safe, I said it was effective! Just kidding, it should be fine to soak them for a day and then blow them off with the air compressor so they don't flash rust. The carbon will be so loose most of it will come right off.