Since you rig is an auto, towing isn't going to be easy regardless of a 3.0 build up or 3.4 swap, the auto tranny's just seam to suck up what little power the 3.0's have. This being said you could do it with your 3.0. I used to tow a 22' open bow ski boat with a 22r, so your 3.0 should do better even being an auto.

I know a lot of people have built up the 3.0 and have been disapointed with the motor cost vs. power gain, however I have been pleased(not surprised) with the modest power gains that I have gotten with my engine mods. My mods up until the Downey headers were a K&N Filtercharger and Jacobs ognition. Not a whole lot of noticeable power, but a 2-3 bump in mpg. The notice in power came when I added Downey headers, a Magnaflow hi-flow cat, and a turbo 2 chamber muffler(had the highflow but it reduced my low end torque to much), along with a complete head rebuild. The cyl's were also bored out .020 after a failed water crossing on Fordyce trail when it was new in '90. My rig weighs over 4600 lbs without gear or passengers, and when loaded I can pass people on the highway when climbing the Sierra's with relative ease, where before I manily stayed to the right and passed very few people, and I am still averaging 17mpg combined when I am not deep into the throttle.

My cost this past spring was about $1500 (headers, exaust, heads, gaskets, t-belt, sensors, etc.,) with doing the work myself. The jacobs and K&N Filtercharger was about another $800. I had looked into the 3.4 swap before doing the heads, and it wasn't feasable. The cost for the harness conversion kit from ORS was going to run about $2K, headers and exhaust work was another $700, the engine was going to run about $1700. It was going to run about $4500-5K to do the job. The $2300 I have invested in mine is a bargain by comparison. I would think that for under $3k you could build a 3.0 to do what you need/want it to for the mean time, as long as you put a tranny cooler and have the right gear to tire ratio. Then again for the $3k you could probably get a used 70's or 80' Chevy/Ford 1/2 ton with a V8 in it, to tow the boat.