Quicky,
THe '75 M1 shares the earlier gearbox woes...
The stories suggested the 2nd gear and shift forks were stronger parts on 'M1.......
The M1 is NOT a bullet proof tranny.
I think they improved marginally in '76 though.
If the bike is to be raced hard...get a good gear cutting engineering shop to undercut the gears closer to 6 degrees.
Sometimes the engineering was so far off that only one dog carried all the load.
Also.....do yourself a great favour and make up/ have made up a Very good case saver to fit around the front sprocket.
When your chain hops off it will totally destroy the clutch operating mechanism.......and thats the case gone too.