Gapping the plugs is a good idea.
Yes, if it runs rich, the plugs will have a lot of crap on them, as well as the o2 sensor.
Coil pack could be messing things up as well, but you'd probably see a CEL for that. Even so, that could be your issue.
Don't shoot the block, i don't think the block is causing your issues. It's a good engine as well