Did you post about your engine rebuild anywhere... Price? Who did it?
I'd guess too, despite the shredded element, that the air intake wasn't restricted when climbing blow-up hill. Mostly because symptoms of a "gagged" engine were not reported.
I didn't follow along every detail, but I don't recall reading about the "last chance" screen at the blower inlet being littered with filter element. Was it? If this was a recent and quick element failure, I would think that there would have been "confetti" galore present.
I wonder too if an over-speed -at some time- could have pushed flow up just enough to be able to suck out the border line element?
The most likely over-speed to go unnoticed would be a downhill run that pushed the engine above governed speed.
How is a 730 in 3rd gear going to up shift if a engine over speeds it has no place to go, they over speed in 3rd being the 4106 has such such low rear gearing.
RJ, what are we calling stock? I am just trying to figure it out using the 3rd gear ratio of a 730 at 0.875 to 1 and a TC 470 torque converter