you dont have to use r31 fronts, pretty much all single piston calipers will keep about the same brake bias

or if you want to get tricky you could measure the r31 front piston size and then can use other single piston calipers that have roughly the same size piston

personally i'd choose what sorta struts you want to use then just go with brakes that will fit onto them

but if you really want to fit r31 front brakes
best way of doing it would be
bluebird front struts, shortened to take r31 or 240z inserts with smaller 1200 sized spring perches welded on or coilovers
then r31 brakes bolt straight on
and you use all the same stuff as stanza struts to bolt them into a 1200, plus a pair of sheer load spacers to properly locate the strut...can get these from datsport

but r31 brakes arent that great from a wheel fitment point of veiw
they are only a 250ish mm disc with a single piston caliper but you need at least 14s to clear them