Subaru LSD's fit.easiest conversion is the older subaru R160 LSD'sall you need to do is unbolt the subaru spline things, and bolt in the 510 output flanges.
The R200 diff's out of Z-cars e.t.c will also go in fairly easily, and can get them in LSD....mostly higher geared tho
to fit them in, you need to modify the rear "mostache bar", and shorten one of your halfshafts.then it will bolt in,
R180's bolt in with no mods.....but LSD's are very hard to find.
or....you could always just lock any diff.....but I'm never one to reccommend a locker on the street