Well, for 1 you can tow more with a manual, and I'm not totally sure, but I think you can tow more with 4x4 as well? Someone correct me if I am wrong. One question for you though, why did you buy the 4.7L if you knew you were going to be doing a lot of towing, and of that amount of weight? I know it may have been an after thought, but just where I'm coming from... you might be up sh$t creek without a paddle.
I'm sure you will be able to two a 6500lb trailer, it just might be harder on the 4.7L and struggle a bit more on grades. Buy a good brake controller and look at a Tekonsha