I think I may have answered my own question. Jumped on exedy.com.au and researched the clutch. Apparently there are 2 different splines in XY-XA gearboxes, a 24mm (standard) and a 27.5mm Heavy Duty box, some. Mustangs and F100s used same box. Just waiting on confirmation from Dad the spline diameter, but I'm pretty sure from the brief time I had the plate in my "labelled" box the slop would've been about right. I'll confirm when I hear back.