Forgot to say-
If you do put in an overdrive gear tranny, or add a Gear Vendors, I would definitely swap out the wimpy 3.55 axle gears, for at least 4.10. A rear locking differential would be a good idea at the same time too.
My dually has 4.10 gears, stock 235 size tires, and the more powerful Powerstroke turbo engine and I have been intending to swap to 4.56 gears, like I did with my other truck. My diesel needs more grunt than the 4.10's are providing. It can still cruise at 65 mph with 4.56's, with the overdrive tranny. F-Superduty trucks of these years came with 4.63 gears, stock, with 5.13's optional, with the same engine, tranny and tire size as my dually. I'm not afraid of going to 4.56. Just lack the money to do it right now.
I also really miss the Lock Right rear differential I put in my other SRW F350. Highway tread duals with an open carrier rear really suck in mud and sand! My next set of rear tires for the dually will likely be Toyo M55's, if I can afford them when the time comes.
What I really need for this truck is a Dana 80 with disc brakes, from an '01/'02 Dodge dually. Rear drum brakes with a 13,000-lb truck really sucks!