The GCWR is easy to find, at least the maximum properly equipped, it is listed in the sales brochure. You are most likely looking at a MAXIMUM of 42k as a GCWR, that is the standard on F700 thru at least the 60's to 70's.
It is also listed in some of the owners manuals.
90 F250 7.5L E4OD, 03 F350 SD auto with the infamous diesel 6.0 for work, 70 C600 330 MT40 & 72 IH Fleetstar 6V-53 MT41 - dump trucks - both are AUTOMATICS! O2 Oldsmobile Bravada, Kubota L3200, Hustler Super Z mower, all Hydrostatics, 85 Honda Elite 250 & 150 scooter CVT - Nary a manual in the fleet!