Go ahead and buy a half ton- fine for a 5000# TT.
But when you change your mind and decide to get a 7500 or 8000# TT,
SELL THE TRUCK AND START OVER.
An 8.1 with the Allison is a superb towing truck with downhill braking. It will be loafing as you blow by all the half ton trucks with the smaller V-8 up 8% grades.
Sure gas mileage is worse but if you are just towing, it might cost a few hundred extra per year. At the most.
I towed a 5000 pound Airstream all over the west in a half ton Chevy.
Replaced the trans at 70K miles. Rebuilt the brakes every other year.