Plugs are not the proble. I had the very same problem on a 380 and never got a good answer to why or what to do.

I do know this adjuster is probably taking up the slack in the chain and that pulled the cam back a little taking up the slack in the chain. I think the adjuster ratchets as it moves out. You can't just push on it to compress it.

I'm sure you've done this but, starting over with the valve covers off. Ensure the crank is pulling the "right" cam with the chain as it comes up on TDC. I'm assuming you call the right cam the one on the right as you look at the car. I think it should be from the driver's perspective, but anyway. The driver's side cam is pulled directly by the crank: as the crank comes to tdc so should the cam come to the mark. assuming #1 compression, the right cam #6 should be lobes down pushing the valves open. Then the pasenger side follows lobes up. Ensure the chain is tight with the cams both at #1 TDC compression. For sure, the driver's side should be right on and the other not far off. All the extra chain is pushed out by the tensioner. One cam should not be right on and the other 1/2 tooth behind.

I'm thinking if I remember right I used one offset key to move the one cam. The gear is still behind but the cam is closer.