My opinion:
1. Most V-8 half ton regular cabs (optioned with the larger V-8 if two are offered) with leaf rear springs and a 8 ft bed and a 3.73+ rear end payload 2,000 pounds and so are a good choice for a Granby.
2. Most V-8 half ton extended cab 8 ft bed trucks payload 1,500 pounds give or take and so are tight on capacity.
3. Ford offers the payload package which increases the regular cab to 2,800 pounds payload and 2,300 pounds for the extended cab long bed (better rear axle, better springs at all four wheels, E load rated tires, heavy duty shocks, all for a only $1,500) last generation. Even more payload on the new aluminum cab models. Very tough to find used Fords with the payload package but look for a leftover new 2014 work truck with the payload package for a good deal, maybe two in the state of Iowa. (My 2013 regular cab payload package 5.0 V-8 work truck is not for sale since I love the truck. 
) Payload package Ford 1/2 tons have a GVWR of 8,200 pounds on the door sticker and 7 lug nuts on the wheels.
4. The newer the 1/2 ton build date, the better the brakes. All models have have upgraded the brakes in recent years.
5. The Ford and Toyota Tundra bed rails are higher so 2002 Granby may or may not need a piece of plywood under it depending on how it was made. Measure to see. If you go 3/4 ton, then the Ford cab is higher so you still need the camper lift if the camper is low so measure the cab height. Granby's with the lift for bed rails and Ford 3/4 cabs have a couple inches of aluminum below the camper door and around 1" without the lift.
6. If you want to do hard off road driving, then get a 3/4 ton. Many standard 1/2's have 2 rear leafs plus the overload spring. My payload package 1/2 ton has 3 rear leafs plus the overload spring. 3/4 tons have 4 rear leafs plus the over load spring. Coming down off a rock with a camper on the back works better with 4 rear leafs. Air bags also work but they have their issues.
7. My 5.0 V-8 gets better gas mpg than a 3/4 ton gasoline. My truck weighs less and my engine is smaller. Expect 15-16 mpg with a light foot and a 5.0 V-8. Expect 10-14 with a heavier 3/4 ton gas truck and larger V-8 depending on how much technology the truck has. Both older trucks and larger engines will lower mpg since engine technology is changing a lot.
8. 3/4 ton and 1 ton regular cab long bed trucks lightly used can be purchased cheaply. I saw a one year old Chevy with 6,000 miles and a 1 ton for $6,000 less than my new 1/2 ton regular cab payload package model. The new price for a 3/4 ton regular cab Ford work truck was the same as my 1/2 ton. We wanted the better fuel MPG's and so got the 1/2 ton. Watch for rounded bottom bed corners on used Chevy/GMC which may require plywood under the camper to lift it above the rounded bottom bed corners for Chevy/GMC based on old posts. Check with FWC on fit.
9. Bigger cabs require bigger trucks. Get a 3/4 ton or 1 ton if you want a crew cab 8 ft bed and say goodbye to mpg for such a big truck in a gasoline engine.
10. If you want to put a 8 ft Granby on a 6.5 ft bed then get a 3/4 ton or 1 ton since so much weight hangs out the back.
