I doubt that you much over 300-320 rwhp with that setup. Same truck, 2001. HX-35/12, DD2's, K&N, Edge Comp. , 4" exhaust: 355rwhp, 775ftlbs torque on a dynojet.
I believe your stock turbo for an auto that year was still an HY35... ..... not much air for towing up long grades. Been there, done that.
I would keep an eye on the temps while towing. I've towed 10k lbs with the above setup for a while without temp problems (come was on 1x5 for towing)
HTH,
George