Agreed...those pistons have "eyebrows", so either too much duration without valve reliefs in the pistons or some borderline-weak valve springs that couldn't close the valves fast enough at high rpm. I suggest you check all the valves to see if they're bent while you have the engine apart.
Not actually sure if the heads that came with the motor was the 1's used, I'm not going to reuse them anyway, a small hydraulic is what I'm thinking depending on what I find out about the cam in question
Uhhhmmmm, yeah. That cam is WAY, WAY, WAY too big for the valves to clear the pistons. I'm sure it started to knock very hard the moment it was first started.
