1996 Chevy Silverado

Leaking intake gaskets is a common problem with these engines. What concerns me about your leak is that coolant was found in the lifter valley and you heard a lifter rapping. This tells me that there is probably coolant in your oil and it probably circulated through out the oiling system.I would change the oil and change it after only 50 miles again;to be sure most of the coolant is gone out of the oil.
New gaskets will solve this problem if it is indeed the gasket. It can't hurt to do a leak down test to be sure a head is not cracked or a head gasket is not to blame. And if you have the ability, once the intake is back on, pressure test the radiator to see if the block itself is cracked. But coolant in the lifter valley is usually a bad intake gasket or head gasket. Let me know how you make out.
Use the fel-pro intake gasket kit these seem to be the best

You can reuse the lifters as long as they do not bind up in the lifter bore and show no sign of wear. Do not forget to prime up the lifters before you set them back in. I hope you are removing both heads. Be sure to check them for any warpage and cracks; especialy around the exaust seats. A long as you've gone this far you mine-as-well go ahead and do a valve job on the heads. Just a little note--the valves in these heads are very durable. You will probably get away with just cleaning them up real good and re-lapping them, but never the less check them closely for pitting - straightness - and wear on the rocker arm end. As long as this motor is stock the fel-pro complete head gasket kit is the best choice for your gasket kit. And make sure you REPLACE THE HEAD BOLTS! Throw the old head bolts away. I think fel-pro has a gasket kit for this motor complete with head bolts, but I'm not sure.
A word of caution though, It is very difficult to just look at a cold head that's off the engine and determine weather or not it is cracked. I would still pressure check the heads or have them magnifluxed by a machine shop, but that's just me. Keep me posted.

Hey, when you get it back together, do a little burnout for me, ok?

If it's a sensor you will most likely get a trouble code. Check your pcv valve. Black smoke is an indicater of oil in the intake or the combustion chamber.