Not sure Andy but an MX900 is 90mm from the centre of the bolt perpendicular to the furthest reach of the brake block bolts, an MX1000 is 100mm, an 890 is 89mm, a 1020 is 102mm and a 1080 is 108mm.....
Put the wheel in the frame, put a bit of tape on the rear stays where the rim reaches, take the wheel out, put a straight edge accross the two tape marks and measure up to the the spindle bolt hole to see which size you need
That's about the only way to have a guess, unless someone has a GT with MX900's on of course
Spen
