the clutch size can be anywhere from 10 inches to 12... depends how the flywheel is drilled. Best to drop the transmission and match the clutch and pressure plate up at your local friendly auto parts supplier.
Thanks for your reply but kind of hard to do that since there is no local US autoparts supplier in the south of London United Kingdom

..
Burbly Dyna
Ps: Surely there must be a specific clutch for an all original 58 1/2ton burb..