Ok so, my friend got F.Moser from 1982 I think, it came with 5 speed in the back. Now, he put 8 speed cassette on it right after he got it and he says he didn't bend the rear fork, it fit right in. How is that possible? He said something like "Cassette dimensions are same, just cogs are placed closer together"
People here told me and I've read on Sheldon's site, its 120mm for 5 speed, 126mm for 6,7 speed and 130mm for 8,9,10 speed.
Moser in question (more pics here):