How wide actually fits to back and how much there is room for backspacing from stock setup? 6cm wider rear fenders, early 944 arms, which widens about 43mm per side with brakes?
Original wheel 4,5 x 15 et34
Kerscher wheel 9,5 x 18 et67 or 73, depends if wheel has flat or high center.
http://www.1010tires.com/WheelOffsetCalculator.asp
67-43 = 24mm
Inner clearance is 53mm less than stock with this setup, probably doesnt fit. Spacers or wheel with less et perhaps? et54 or 60 is next smaller et.
But again, base point for these measurements could be perhaps typical empi 8-spoke which is 5,5x15 et 30 and it fits under stock rear fenders easily. At this calculation, 9,5" et67wheel is still used.