We have a 2006 GMC 2500HD with the 8.1, Allison 6 speed. The tires (Michelin 285/70/17) that we currently have converts the axle ratio to approximately 3.42. We do have air bags and use an equalizer hitch.
Our camper is an Outfitter Apex 8 and weighs about 3200# loaded for a trip.
We tow an H&H 20 foot car hauler that weighs empty about 3900# and has a GVW of approximately #9800 lbs. Our usual loaded weight is about #8500 lbs loaded.
I keep up with traffic going up Vail Pass. Generally staying in the right lane and passing some slow traffic when I need too. The Allison will be in second to fourth gear depending on conditions.
I have had no troubles with the truck. The truck is in good condition.
Your Ford will can do the same. I doubt if you need any gear change or 19.5 wheels/tires.
Good Luck.