Bouncing back to this thread...don't overlook a hydraulic roller if your going in that direction. I run one with hydro roller lifters in the race car and am very pleased with it.
No slop in your timing chain ? Cam timing retarded/advanced ? I would think if the valves were not closing(weak springs) you would have heard them floating something awful before catastrophic failure.
Back to a binding issue ,get a straight line measurement of a bent rod vs. true one and subtract the difference. How much are milled from the heads, gasket thickness ,deck height ? My guess is pushrod length obviously wasn't checked in assembly. Again a real mystery to me that motor ran fine for awhile before breakage Geometry problem.
No raw fuel in the headers ?
What was your run time? Was the initial "tick" immediate upon oil pressure coming up? Was it primed before start up ? I continue to find it hard to believe it was broken in prior to your purchase.
Goes without saying use an adjustable pushrod to obtain proper length this go around. They will vary with machining/cam/rotating assemblies which deviate from stock measurements. Sometimes per cylinder, per I/E. I'm into bigger cube small blocks but has to hold true for " the heavy engines" also.
I would have bet shrapnel made it's way to the bottom of the pan and "machining" to the filter--good for you. Keep us updated on your findings.
(hydro roller/roller lifters, fresh/matched valve train, low maintenance ground pounder with good vacuum and idle characteristics IMO)