basically i know there is a certain limit that the manufacturer suggests that the vehicle can tow. What I'm wondering is if I increase the engine output, add more leaf springs to the rear suspension, and upgrade the brakes, will the truck be able to tow 7000 lbs or more?

yes as long as the trailer has its own brakes you can tow much more than manufacturer recommends.especially if you do those upgrades you mentioned and you may want to lower the axel ratio to put less strain on the rest of the drive train.