R31 struts are the 2.0 inch size. So you need a steering arm for a 2" strut.

Get a B310 or A10 steering arm ... but beware, they will make your steering slower.

Will 1200 ball joints fit into the larger steering arm? If not, then you can fit W50 ball joints. Which will push your track out a bit and add some negative camber.

The best thing to do is modify a 1200 steering arm to take the larger strut



See Knuckle Arm