With the weight you are towing I'd keep the power where it's at. You will run into EGT issues with anymore fueling than you have now. NV4500s are weak in several areas. The stock 1 1/4 input shaft can break with a with extra power and a stout clutch. If you are going to a 13" disc, I'd upgrade to a 1 3/8 input shaft as well. 3rd gear and 5th gear are also common problem areas as well. Later style head gaskets have thicker fire rings and are better.