My personal opinion is that you're wasting your money if you purchase a replacement spring that is designed primarily for off road use (i.e. soft ride and travel) when you're planned use is towing.  Even if they can "handle" the load without going flat, the decreased spring rate will force you to run extrememly firm shocks to avoid bouncing down the road.  This can become a handling issue related to safety.

If you get a Superlift/Pro Comp type of lift the springs maintain a spring rate that is closer to stock.  The majority of people that have problems with this type of spring use them for more aggressive off roading than they were designed for.