Parts washers are actually really cheap if you look around. I've seem them under $100. Then you fill it with mineral spirits and one quart of tranny fluid so your parts dont all rust as soon as you dry them off. Saves the enviroment and pays off in the end seeing you can wash many things in the same solvent over and over again.
BTW, WD-40 was not made to be a degreaser...but rather a water displacement solution (Water Displacement, 40th attempt). Because one of the main ingredients is mineral spirits it actually works quite well as a degreaser. If only I could fill my parts cleaner with 20 gallons of it!
Oven Cleaner is safe on the metal, I use it on plastic models when I screw up the paint and it is even safe on plastic. It will definately remove paint so plan on repainting those heads. Brake clean and compressed air make a great combo and laquer thinner is also a great degreaser. It may be worth it to take it to a machine shop and have them dip them. Prolly not any more expensive than brake clean and you dont have to worry about the environmental impact as they have proper disposal methods. Just a thought.
Another vote for the machine shop. I was thinking that from the beginning of this thread. I think the one I went to charged me $25 to hot tank my heads.
Harbor Freight sells a parts washer for like $60-100 I think. My question is, what do you do with the solvents in the parts washer once you've used them a lot? I guess you'd take them to your local chemical collection place...we have one ran by the city, but you have to call and make an appointment first.