Where is the water leaking from?
The information you are getting is correct about radiators and thermostats.
Do you have any idea why the truck was parked in the first place?
I have a suspicion, and it isn't pretty.
One of the old time fixes for a motor that overheats (and only in low pressure systems) was to remove the thermostat and/or the pressure cap. The reason that they did this was usually because the block was cracked. Any time you tried to pressurize the block, it would start leaking and overheat quickly. Removing the thermostat/pressure cap would allow you to run the vehicle quite a bit longer before it overheated - but it still would eventually overheat.
What you need to do is to run the motor with a pressurized radiator, but make absolutely sure you know where the water is coming from when it overheats. If the block is cracked, you should start seeing evidence of where it is. It can also show up in the exhaust as white steam, or in the oil as a sort of chocolate milkshake, or possibly white globs on the dipstick.
