depends what gearbox you are using as to what ratio is good. I have a 3.4 in my E30 which is short for the ZF gearbox but its not a road car.
Z3 have the same output input and output flanges as E36 so it all bolts in, wheras E30 you need to change the output shafts. What drive shaft are you using if it is the 328i one then E36 input shaft on diff.
E30 one looks similar but different
Beware of the Z3 LSDs, they are not viscous type, might be fine for a road car but they are non-rebuildable and cant be tightened up like the E30 ones. My Diff is all Z3 except with an E30 viscous center rebuilt and an extra clutchpack installed.
hope this helps