This is a great post! I took the recent TorqueFlite A-727 book out to see what it said about the Belleville spring because I don't remember anything about strength being mentioned as John Kunkel pointed out. And I was right it wasn't mentioned. But here is an excerpt of what was mentioned:

"Unlike the front clutch pack, the rear uses a Belleville spring to return the piston upon release. The Belleville spring was changed in 1974 to improve the neutral to drive shift. At the same time the height of the piston was increased. THESE PARTS DO NOT INTERCHANGE WITH PRIOR MODELS."

If John Kunkel is correct and you have a later piston and early Belleville, and since they don't interchange, could this be the reason for failure? (I know Kunkel alluded to this). Sounds like the builder may owe you another 727!