My first car 72 Scamp

Car has been smoking out the right exhaust real bad. I’ve been driving it a little but not much due to it smoking and getting worse. The two pics of the plug that is gunked up is #6 cylinder. The other pics are the others on the same side. We’re thinking likely bad valve seals and possibly head gasket. Gonna do a leak down test and compression test. The top of the piston is caked up. Hoping nothing wrong with the bottom end. May end up using this as a chance for some 302 heads.

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Quick temporary fix, use two heat ranges hotter plug