Ahh, M42 is a much simpler proposition. Profile gasket should be checked out while its out though and might as well do the cam chain tensioner. Cant help much more though, i'm a six pot boy
E30 diffs are the same across the range with regard to mounting.
They come as either small or medium case. Bigger obviously stronger and fitted to the larger engined cars.
Favourable ratios or LSD (for a 1000kg BM) are getting a bit pricey now up around £250-350 but i dont know what sort of ratio you need in a roadster (not started building yet)
Mount with four large bolts on top through the rear beam, and a large bushe mounted 'ear' to one side. (this can be replaced with a Z3 bush for stronger mounting)
Cheers
Chris