LSD is not the determining factor. A small case, stock 318is diff will bolt up no worries.
I just installed a 4:10 LSD out of a 325 vert med case. I tried to installed it twice, it seemed like the drive shaft was too long. The med case is slightly longer than a small. I had to loosen the slip joint on the drive shaft and shorten it by 2cm or so. After than the med case bolted right up.