The MSA kit is made by Precision, which is the same stuff everyone sells. There was a difference between the 280Z kit and the 240Z kit, judging by the archives, with one having thicker or stiffer door rubber. At some point, they started selling just one kit. My guess is that it is the one they sell is the stiffer one.
Here is something interesting I came across a few days ago: I was looking in a old JC Whitney catalog and ran into one of their ads for weatherstripping. They showed cut-way views of the different patterns and it looks like there are more than a few that could be good for the Z. The main thing to do is go to a good junkyard and start pulling weatherstrip from a number of vehicles (newer is better), try to match up the Z's rubber, and go from there. Can you imagine finding good, soft, pliable rubber that would work for the doors?!? Yeah, I can't either.
Davy