I now live in farm country, and you would not believe the stuff that 3-cylinder farm tractors that only generate 60 - 75 hp manage to tow through axle-deep mud every single day. Towing a relatively lightweight (under 2000 lbs) boat like yours on a trailer over a nice smooth road is nothing compared to what these things deal with. They will literally pull small trees right out of the ground with their humble, 2.9L 3-cylinder engines.
The number of cylinders, or even the horsepower, doesn't matter. What matters is torque.
It also matters how you measure torque. Lots of car companies (Ford being one of them) require you to use premium fuel to attain the torque levels they claim in the specs, put in regular gas and your output decreases.
I have no idea if a new Bronco is a decent tow vehicle or not (die-hard Toyota guy here). But I can tell you 100% that you're looking at it the wrong way ... cylinders and hp don't matter. What does matter are torque and the drag of whatever you're trying to tow. For a boat like yours, you'll want something with a powertrain designed to haul loads, not to run around town while achieving maximum fuel economy. Maybe that's the four-door Bronco, or maybe it's a Ranger (or better yet, a Tacoma).
Crowman's comments about wheelbase are also spot-on. It's not just ride quality, there's a real safety consideration there.