update- I was right (this is my "told you so" moment )

They pulled the heads and found some deep dish pistons. I wish they had taken a measurement of how far down in the hole it was, just to satisfy my curiosity, but oh well... Anyway, after sufficient prompting the truth came out. According to the story the parts supply warehouse could not find the correct 413 .030 pistons for a car as specified in their catalog but they had some 413 .030 pistons for a truck on the shelf, so the parts picker guy figured "same thing, right?" Eh, not so much. According to the book this piston yields a 7:1 compression ratio with closed chamber heads, so with the 346 heads they had on it I'm impressed that it ran at all.

Hope this helps serve as a cautionary tale to anyone pondering the rebuild of a 413.... be careful what parts they try to sell to you.