I believe these plastic pieces are your valve guide seals. They break up like that and can block the screen. Small black brittle pieces. I've seen this before. You don't have to pull the motor. Puff of smoke when it starts? just my thought.
I have a 440 pretty much stock 68 motor in mine, and my garage is probably worse. Been trying to neat up for years .67 Dart convertible,5 speed. Your car sounds great.