what size truck do i need to tow a 26' trailer

I towed about the same weight with a 2007 Silverado (5.3, 3.42 gears) and it towed OK on flat open road but it was a total dog going up any hill even in the low elevations of Missouri. Given that, I doubt you will be happy towing that much weight with a 4.3. If I had to pick an F-150 to tow 5700 lbs, I'd look at an ecoboost otherwise, I'd look at a 2500.