A stethoscope will definitely help narrow down the noise.
The backfiring could be sign of improper timing, a sticky valve, or a worn or stretched timing chain and especially if it's a high mileage engine. The irregular knock might be the timing chain hitting the cover but I'm not sure about this. It's unlikely and I can't say from experience if this is even possible.
The easy way to check the timing chain is pull the distributor cap, put a socket on the crank, turn the crank back and forth, and watch the rotor to see how much slop is in timing chain. The should be some slop but not an excessive amount.
The next thing I'd do is check the ignition timing and then do a compression check on all cylinders.
This is a stretch but I'd add some Seafoam to the oil (6 ounces) to see if that might help free a sticky lifter or valve. It won't hurt but then again it may not do anything.
If you want some more expert advice go to fullsizechevy.com and join up. There's some very knowledgeable chevy gurus there. Here's the link ...
http://www.fullsizechevy.com/forum/forum.php 
