I had the same issue.
I shaved .5mm of the the collet by sanding it on emery cloth making sure it was taken off like it was machined off.
It did the job, I just get .3mm endplay, with .4mm fly wheel to crank case clearance.
I have a GRP .21 and only have .4mm of clearance with standard GRP collet.
Not sure why I was the only one with the problem because I can install clutches blindfolded.