IMHO, if you have one of the 6 sp autos, the axel ratio is not as big a deal as in the past. Many rigs with 3.42 ratios, are better off than my 4.10 gear rig from an overall low ratio factor to get a load going.
As far as payload goes, 6500 lb trailer will have 700-1000 lbs of hw depending upon the how it is loaded, plus people, My case 1200 lbs in a family of 6.....add to 1000 lbs of potential HW.....I need 2200 lbs of payload. A 15 series truck will not be strong enough payload wise. If it is just two of you, may be 4, then a 1600 lb payload rig may be enough if you keep the HW to the 11-12% range vs 15%......Get the largest HP motor you can......Enjoy!
marty