it DID work!
Here are some photos:
So here's my 14060M Sub with the standard Sub caseback:
Here's the Sub caseback:
And here's how it measures, just over 1.5mm:
Here's is the GMT caseback I sourced:
And it measures just under 0.9mm...wow!...pretty thin...
My watchmaker said it went it without any issues and he swapped the gasket and pressure tested the watch to 100m
Now it's a little slimmer, similar to the 5513 which is pressured rated to 200m instead of 300. I doubt it will ever go past 10 feet. If I ever sell it (doubtful), I will obviously fit the original caseback on again....