In my Pantera, all of the issue is in getting the relationship between the throwout bearing, the clutch fingers and the slave cylinder to where the release is correct.
In the Pantera it isn't easy to measure the clearance between the disc, the pressure plate and the flywheel but when I set up clutches, that is a key number. You need to do that with a feeler gauge.
The diaphram requires less clearance at full travel then the Long type does.
You are going to ask me what those numbers are. At this moment, I don't honestly remember. I'd need time to come up with those, but those are the key numbers.
I do remember that with the original Long type that I took out of there, the slave was maxed out. The diaphram provided more clearance.
Why the Centerforce is spinning like a dumb-dum for you is inconsistant to what I have with them.
I have a Centerforce in my '68 GT350 and my '73 Pantera. Neither has any kind of issue but as Shirlock Holmes once stated, "once you eliminate all of the obvious, whatever remains must in fact be the answer". No kidding right?
What did you do with the pilot "bearing"? As per the pilot bearing discussion, there is a maximum tolerance allowable on the wear. If you want to be comprehensively scientific you need to look at that also.
Using Marlin's numbers on that would be highly advisable,