I'm surprised to see counterweights on your old pressure plate. The pic below is from an original Pantera pressure plate. Has your bell housing been "clearanced" for those counterweights?
Or, is your old pressure plate a 10.5" instead of an 11" ?
I wonder if the PP finger lengths are different between the two PP's ? A slightly longer finger (measured from the center of the pivot) would require more "throw" from the slave cyl.
John