ring is smaller/weaker.
spring perches are for coils
axle tubes are smaller
brakes are small

width might be close.. don't know. You could make it work if you wanted to, but I cant think of a reason other than it's free. It might end up costing extra though.

I think some people have put the rear discs onto 1stgen rears, but they have kinda small pistons. You'd have to do something to get some more work out of the rears.