Closed chamber with flat tops

Sounds more like it was overreved. The lifter clips do ocasionally come out. I've seen it on several brands although the most consistently are the ones from Edelbrock that use a flat steel rather than round wre clip.
I would have the valve springs checked - if it's a 340 replacement cam and they left the 318 springs there's your reason for the clips popping and piston "eyebrows". If should fit with no problem, but like any cam, the valve springs have to be upgraded to match the cam and they need to be set up properly to give the right pressures.