'68 318 - Coolant left in heads when removed.
Yep, Core wires. Like "WTF" when you find them for the first time.
Yes wireslike bits of coat hanger. They are used to position the casting cores. The iron forms around them and they stay when the casting sand is removed. Eventually they corrode at the ends and end up in the bottom of the block.
The crap in the bottom is generally rust, minerals from tap water and possibly silicates from antifreeze that has not been changed semi regularily. Highly recommend to get the pipe plugs out. You may have to use an acetylene torch to heat them and left them cool to get them out. If it is built up fairly deep when you get the plugs out and clear the hole, it may be adviseable to pull the core plugs out or at least one on each side to check for corrosion.
Years ago a guy dropped his car off for some work like tuneup and carb kit. He went to work up north and said he would be back in a month. Well he got back and came to get the car on a Saturday morning. He left the lot and got about a block down and steam came out under the hood. I just happened to be there to get some of my tools to work on my car at home. He came in mad as a hatter saying he left the car to be serviced so he did not have problems. I had not worked on it before but was there so got roped in to deal with it. What had happened was small core plugs on the lower outside edge of the head, just above the head gasket had corroded and just by coincidence decided to spray on the exhaust manifold. I got the offender out and went on the hunt for enough to replace all those in the heads. These were in I believe a 383 and were a bit of an oddball size. I went to 3 parts suppliers who did not have them. In Edmonton in those days most shops closed at noon on Saturdays. Finally found some at 5 to noon. By the time I got them all in, my car did not get its TLC.
Just saying, preventive maintenance even if it is a royal pain in the *** is better than needing Dr Hook to get home.
