There's not an upper limit on the number of miles you can put on the truck. It depends very much on how it's been maintained and how heavy the abuse that it's taken has been.
Some owners put several hundred thousand miles on cars like this between engine rebuilds. Just depends on how it's treated.
Hi I had a light 3/4 ton 1988 GMC . Drove the shit out of it on dirt roads carring big loads, Had very little trouble, never touched the engine, I sould it with 525.000 km on and as far as i know the guy still uses it