you will never break a r31 diff in a ke70.. never
yes you have to change the mounts over from the ke70 ones. the diff gives you 30mm track increase each size so fitting descent wheels under the stock guards is not gona happen.
but dont bother with the 4k, waste of time.
with the 31 coilovers you need to modifiy the top mounting point. cutting the current one out and welding in a new plate big enough with the correct holes is what need to be done. you will need to check the length of them, that will determine where they are mounted.