I believe the GRP3 brakes are similar to the later Pantera brakes. My car has those bigger brakes and I can run 15" x 8" all round without any wheel spacers. Attached is an old photo, the calipers have all been plated with new stainless pistons and seals. These brakes were used on many other cars, Aston Martin, AC Cobra, Masarati going by the pad equivalent charts, maybe different spacer shims were used between each caliper half depending on rotor thickness.
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