There is no accurate way to answer your question short of putting it on some scales.
Some states require a weight slip to register a truck like that and I've seen scale weight on the registration papers. Its possible it was weighed before the guy you're getting it from purchased it and that weight is listed on the registration.
Otherwise Chevy says a 6103 weighs about 4700 pounds probably more before you add a bed. GM didn't make the dump bed and hoist and unless you can identify a manufacturer and get a weight from them weighing the truck is the best bet.
If I were guessing I'd put it at upwards of 7000 pounds assuming its a heavy steel dump bed and a big hoist to dump it. If he is putting it on a trailer and the trailer isn't a tandem axle with 8-lug wheels or maybe a lowboy type trailer, its going to be too heavy.
