* The sticker on the driver's side doorjamb: This sticker will list the recommended tire size(s).
* Your owner's manual: This will also specify the factory-recommended tire size.
* The sidewall of your existing tires: If you still have the original tires (unlikely after 27 years!), the size is printed on the sidewall.
Without knowing the specific configuration of your truck, I cannot give you a definitive answer.