It should bolt into the frame around the motor without any problems. The linkage is where you might have problems.
On my bike I used a 92 frame, 87 swing arm with the 92's shock. I had to make some spacers for some of the linkage. I'm not exactly sure what year linkage I ended up using (put them on the bench forgot which was which when I came back a few days later
). I do know that out of all the combnations I tried, none was a bolt in deal without some kind of spacer needed.
Although now that I think about it, if I needed spacers, that means the 87's was wider around the piviot arm....meaning the 91's would be narrower. The only place I needed the spacers was for the dogbones on the piviot and the piviot to frame brackets.
Hope this helps and doesn't confuse you.....I know I'm sort of consfused now.