If all eight of the plugs look exactly the same, I'd be leaning away from the issue being with the intake manifold gasket and valve seals.

First and foremost, as mentioned, perform a leak down and compression test.

What I would be thinking about now is did the machine shop set the gap correctly for the top piston ring. Are those pistons hypereutectic? If so, they require a different gap than cast and forged pistons.