What is the towing capacity of a 1983 GMC, K-Series, 3500 box truck?
Before you go too much further make sure the truck has a trans cooler on it that other than the radiator. If it doesn't get an aftermarket cooler from the parts store. Really simple to install and remember the vehicle will also take extra trans fluid with it installed. With that said try calling a Chevy dealer to get the towing info.
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