dd
a lot of aussie built cars had aussie components due to our "local content" import laws in the 70's/80's
diffs gearboxes and brakes mostly

r31 diff would be pretty much the same weight as a hilux one with discs once cut down

main reason people go r31 over hilux is that with the hilux one you cant reuse the hilux axles(not long enough to cut and respline) so you need to find other axles to suit or get billet ones made up

and 4 stud :)

yes hilux axles neck down after the spline so you need to cut enough off to get to the thicker part again
depends if your willing to change wheel offsets
all the good brakes are 5 stud anyway