The best thing I found is OVEN CLEANER. Here is what my 100,000 mile 302 gunked up greasy motor looked like after 6 bottles of oven cleaner, 2 bottles of spray paint, and a garden hose.
I took my Comet on a car dolly to one of those do-it-yourself car washes after I pulled the engine and trans, wire harness, etc. Used engine degreaser and oven cleaner, pressure washed it all down. It wasn't paint-able right away, but I did get 90% of it clean. Those car washes have to separate and recycle water, so a little grunge didn't hurt anyone and I made my mess somewhere other than our house. For a handful of quarters, it was worth it to me.
