Did this engine give any indication that anything was amiss in the 400 miles you put on it, were you chasing a perceived valve train noise and rechecking lash etc, did running quality drop off during this time gradually or suddenly, did it ever sneeze or backfire on startup, has the retaining bolt for the front damper ever been found loose or the engine turned over with it loose or not fitted.
The reason I ask is in your gallery you show a pic of the engine with external
oil pump, during setup with the heads & valvetrain in place,was the engine EVER turned over(cycled) with the damper bolt not fitted? As someone earlier suggested this could have resulted in a partial shear of the crank key and resulted in the cam timing becoming retarded to cause the damage you have here. I had this happen to me on my old Boss 302 race engine while on the engine stand lashing the valves, fortunately I actually saw it happen, In your situation you would not have been able to notice. I feel that you have to recheck the cam timing before further strip down & recheck your valve to piston clearance. I also note that the engine had been run prior to fitting, was this on a Dyno or Test stand.
Jac Mac