Rust behind freeze plugs
 

After cleaning out the block with a hose, I recovered at least 1.5 cups of "rusty sand and gravel" out of each side of the engine block behind the freeze plugs. I'm stunned this engine didn't overheat and cook itself. Then I removed the oil pump and found a lot of debris clogging the pick up tube. I would like to know what this stuff is. It's chunks of hard black plastic (almost like gravel). The chunks of clear plastic teeth are a mystery too. There was so much sludge in the bottom of the oil pan surrounding the pick up screen. It was like chocolate jello. When I drained the oil, it would stay in there. With the screen clogged and the sludge smothering the pick up tube, it's a miracle the engine didn't seize long ago. The timing chain was loose, so I changed it. I ordered a new oil pump/screen. Sometimes you don't know how bad the inside of an engine is.

timing gear was made with plastic teeth

I've torn down a few seasoned engines and found the same. The jelly like goo is the result of running engines too little to burn off condensation. Sometimes it is from going too long between oil changes, say like 20,000 miles and more!
I rebuilt a 360 from a 78 Cordoba that had a huge chunk of ash type material the size of a dinner plate, STUCK to the bottom of the intake manifold. This was the first Mopar engine that had to be bored to .030 to clean up the bores. Other engines I have bored them to .030 because standard bore pistons weren't available.
I didn't see what engine you referred to. I don't think Big Blocks ever came with nylon coated gears though.

My 76 was the same way, lots of crap out of the freeze plug holes. The black stuff I bet is valve guide seal pieces. I need to check my oil pickup too.