OK, I think I figured out the problem. I believe I will receive the dumbass of the year award for this one.

I never removed the blue shop towels from each intake port before placing the intake manifold back into position.

Yes, you read that right. Each intake port has a blue shop towel in it - not real towels, but the thin ones that come on a roll.

I will pull the intake and hope that there's no damage to the intake valves as a result.