If your good, a 5-speed theoretically would get better gas milage. I still wouldn't tow in 5th though. I've noticed lately that vehical tow rating are now higher for auto's. I have a chevy blazer w/ the 4.3L auto. tranny that tow's my 93 SN just fine. I usually pull in 3rd unless I'm on the flats and at high way speed. The difference between 3 and OD is about 1k rpm. With an auto you just have to make sure that you are not shifting up down or with the torque convertor unlocked alot or you'll burn up the tranny. Also w/ and auto you should have an external tranny cooler and change
it's oil at least every 30k miles. With the 19.5' SN I'm probably only towing 3000-3500 pounds. If I had a larger wakeboard boat, I would probably opt for a v-8. My 4.3 has about 250 ft-lbs of torque, many other v-6's have much less. Make sure to take that into account when deciding what vechicle and how you are going to use it.