Destroyed Thrust Bearing in Newly Built 327

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I am far from a frequent poster but I hope someone can point me in the right direction here. I built a 327 to replace the original 305 in my 1978 Custom Deluxe which had a newly-rebuilt TH350 transmission. I turned the engine, with a new converter (2400 stall) and flexplate, to a local mechanic for installation. An oil leak that developed during the first approximately 2500 miles, finally led to pulling the pan to find that the thrust bearing was badly damaged and that debris from it had reached the rear seal, causing the leak.

The engine is in my garage now and the truck is at the installer's shop waiting for a repaired engine. The crank is ready to be lifted out, but is still in the block. Closer inspection reveals a heavily worn rear thrust bearing flange as well as very heavy damage to the thrust ring on the crank. There is virtually no wear on the forward flange of the bearing. Being that the damaged bearing flange is the rearmost flange (corresponding damage to thrust ring on its forward-facing surface) it appears that something had been pushing hard on the rear of the crank. The installer is sure that the converter was fully engaged with the transmission and that appropriate end play existed in the converter before it was bolted to the flexplate.

My questions to the group are, given all of the above, is there anything related the build that could, if done improperly, could cause the above damage or, is the damage definitely due to externally-applied force? If the crank is being forced forward externally, what would you suspect? I am really concerned about repairing the damage only to have a repeat performance. One final observation; the pilot recess in the rear of the 327 crank is significantly shallower (approx. 3/16) than that on the old 305 crank.

I apologize for the length of this post, but I was trying to provide all of the info that I thought was pertinent. Any input from you folks will be greatly appreciated.