How to get the gunk off?

The father-in-law and I put in some good hours on the Dart this weekend. Changed the trunk seal, replaced a broken stud on the back wheel, replaced the valve cover gaskets, which were leaking like a sieve, and rebuilt the carb. I also slid under the engine to see if there were any other leak spots. Besides the oil pan drain plug leaking, I couldn't see any other leaks. What I did see was about 15 years worth of oil, grease, grime, and who knows what else on the exhaust manifolds and the bottom of the motor. What should I use to get all this gunk off, so my car doesn't smoke like crazy as soon as the manifolds heat up?

Thanks for the info!

DaveZee