Okay, everyone, here's something you need to watch out for. My '99 4.2 blew a head gasket, let water into the right front cylinder, and bent a rod when I tried to start it.
I rebuilt the engine, and it ran like a top for about 3,000 miles. One day, I went to start the engine after it was hot and had been doing some shopping, and it made bad noises again. The same head gasket had blown again! Fortunately, it didn't bend a rod this time.
Turns out the right side catalytic converter was clogged. It caused excessive back pressure, and that caused the head gasket to blow.
If your truck has high miles on it (mine has about 270K), be aware of this and watch out for it!
To check it out, take out the O2 sensors on both sides, put pressure gauges (15psi) in each hole, and start it up. At idle, you should see little, if any, back pressure. Rev the engine - both sides should go up the same, and shouldn't go over 3psi. If you see more than that, replace the converter. It's a PITA, but it'll save you some real grief down the line!

 

Muffler shops have infrared testers that can detect clogged cats without having to pull anything apart.