Re: First time boat buyer, need some advice.
Depending on the year, GM 3/4 ton pickups should be able to tow about 10,000 pounds. The owners manual should give you the answer. With a rebuilt tranny and a good trans cooler, you should be OK if the boat does not exceed the rated capacity of the truck. Also check the rating of the receiver unit mounted on the truck; many of the factory units are not very heavy. You also likely will need a load leveler/equalizer hitch, and good brakes are an absolute necessity.
Some of the older GMs did not have transmission temp gauges; if your truck does not have one, install one and keep an eye on it. Does it have a tow mode button on the gear shift? If so, also use that.