Help please, plug reading wizards!

RGAZ said
#4 and #6 are being fouled by oil intrusion in the cylinder. Likely due to valve guide seals. Pull your cover and inspect the seals.

RGAZ said
He stated: "Sadly, I am the goon that assembled this engine and it was my first motor rebuild."

So I am taking a shot that it was not done correctly.

Good idea, I'll check that out. The head was done by the machine shop and it's a good one so (hopefully) that's not an issue.