If you filled out your Profile you would get more definitive answers, still wrong but definitively wrong.
What state is the rig in and do you have it in bits or is it whole?
If the vehicle is complete and untouched the clutch will probably not disengage. It is stuck from 20 years of inuse. If it runs and can move you can free up the clutch with some brute force. I would put it in low range and start it in gear, then when it is running forwards push the clutch, accelerator and handbrake (footbrake is aksing a bit much) togather. The engine force will sometimes cause the clutch to lose the grip and start spinning.
What you can do to the gearbox depends on whether it is in the vehicle and which vehicle it is. I woudl at least try to take off the shifter boots and panel around teh gearstick ad remove the top of the box. This will allow you to see if it is rusted solid or just seized up from years of sitting there. If the latter you could try some very light oil to penetrate the gummed parts and if it frees up change it.