If you can use 8.6, I find little point on installing 8.1... why not leaving 7.5.x (or 7.6.1, especially with large partitions on the disk) on a partition? These machines fly on System 7. If you ever need some 8.x features (HFS+ volumes is a good reason) you can go all the way to 8.6 -- in my experience, the best compromise between functionality, stability and performance.