So the saga of my Century Club 671 gets even better. Rob's post above with the sub for the 8404-60 part tickled a brain cell. I have an 8404, so I took the pilot screw out of it and sure enough it fits. Since the 8408 is far from pristine, and the 8404 part matches my CC 671, I will just use the shiny replacement in the 8404.
So I ran the engine some more and it still wasn't acting right. The sloppy truck was silent but the motor still seemed unusually noisy. I lubed it before I used it but I did not replace the original Lionel lube that was in the gear box. I decided I would remove as much of that as possible and re-lube it with lubriplate. As I was scraping out the old grease, I found a lock washer mixed in with the grease. At first I thought I displaced a shim from somewhere but as I cleaned the washer and realized it was a lock washer, I knew it wasn't a shim. It appears to be the exact same lock washer as is used on the motor mount screws. Nice huh? Sure glad I found it before it chewed up the gears...
You never know.
Tony