It's the size that's the issue (according to the guy I talked to). Chevy lifters have a .842" OD whereas Mopar lifters are .904" if I recall. I would assume the OD of the snap ring groove is larger on the Mopar as well.
I'll show a pic later. The snap rings are pretty close and the crower ones are, in my opinion, far less likely to self eject from the top of the lifter in any case.