I doubt that a K4 PG1 is strong enough to be fitted to a KV6, but a diff (Torsen or superior Quaife ATB) is a good mod to have anyway.sorry, i forgot you were k6 slags
this will bolt to any k (4) series engine, and yes as pointed out above, if you have a 1.4 or 1.6, you will need to change starter - mount - shafts at least. however, as i said it bolts to any k (4) series engine.
BRM gearboxes use a type B diff, a 3.9 final drive with a close ratio gearset.
you could, build it into a kv6 box, or simply swap the diff into it.