Thanks to Tom S for going to the shop and documenting things with the machinist.
I just spoke to the machinist. Yes, there was some sort of crank radius problem or bearing failure on #5. Friction, heat, seizure...
He measured endplay again and it was perfect. 6-8 thousands I think he said.
He says that no regular guy can be expected to see or know this would be a problem.
He is going to take all sorts measurements of the Scat crank vs. a stock Pontiac crank radius and those bearings to let me have technical info on the specifics of the failure.
The crank is repairable with a 10 under grind. The radius (fillet) will need to be reduced or a different bearing used that is narrower or has an appropriate chamfer.
The block is likely salvageable, but will need some work. Yes, it looks like a chunk broke off in the picture, I forgot to ask. That is for Monday.
Regarding cleanliness of my assembly area. During assembly, I was constantly texting him with questions and photos. He noticed that I was working in my garage and sometimes in the driveway and didn't like that.
I agree, but had no choice. He doesn't believe dirt or debris played any role in this failure.
I'm waiting on Monday, then I need to contact Butler with the info about the crank and/or bearings and see where I am.
Frankly, between the machinist and Tom, they've communicated that I didn't f**k it up, which is a relief. I can live with myself.