What is the gvwr of a 1989 ford f600?

20,200 lbs. I hope you don't have the Lucas Girling hydraulic brakes with the transmission brake on that thing... they're horribly unreliable, and you can't find parts for them. Good news is you can always swap in an International rear axle with the regular hydraulic brakes, although you will have to install and mount a hydroboost booster to make that swap.