it can fit ... final drives are the same ratio ...but you have to split the case to put it in ... so its big job ...yes the 5th gear on 80-81 are the lowest of all oldwings ... 82 5th is like 1000 5th but 82 gear spread is much better than 1000 is ...83 is taller yet ...and the gear spread is as good as the 1200s just a little lower final drive than 1200s making them taller overall