You are definitely pushing the limits at 5600lbs with the 3.42 gears. I wouldn't hesitate towing the boat and trailer 15 miles but I wouldn't do 300 miles unless its all flat ground, you have a transmission fluid cooler and the TB is not completely loaded down with gear. I've pulled a friend's Four Winns and trailer weighing about the same as your boat/trailer without any issues and I have the 3.42.
Thats going to be an issue, if you have the 5.3 you'll be better off than the I6 which is going to struggle especially with the 3.42s and that much weight. Definitely invest in a trans cooler and maybe a trans temp guage, Id also switch to synthetic motor oil to help the engine on the long trips. The 15mi trips would be easy to do stock, but the 300mi ones are hard on it. Take it SLOWWW, thats a lot of weight to stop, starting is easy stopping is the hard part. My boats are 4000lbs and even with the 3.73s my 02 I6 has to really work on the 330mi trip from seattle to coeur d'alene up a 3000ft pass from sea level. Trans and engine stay nice and cool, I use synthetic oil, diff fluid and have a trans cooler.
You'll be fine going 15 miles, and if you're on a relitively flat highway for your 300 mile trip, it'll probably be fine, as well... The only place where the gears really matter is from a start, and on hills... I wouldn't pull over a mountain with it, but if you're in the midwest or the south, you'll probably be fine...
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