That is a beautiful truck and I'm sorry to hear about your plans for it.
Reasonable gas mileage: If you want to achieve more than around 13-15mpg, you'll need a new engine, transmission, and rear end.
Not interested in speed: The truck will cruise comfortably as-is at 55-65 mph.
Lower than average maintenance: What vehicles are you taking in to account to get an "average" maintenance number? I use my truck all the time and I do almost zero maintenenace on it at all. I greased the suspension last week. That's the only thing I've done on it in the past 2000 miles.
Frankenstein-looking truck: An AD truck with a modern frame, modern drive train, modern suspension, et al is the very definition of a Frankenstein truck.
There are plenty of AD trucks for sale with the modifications you describe already done. Why not buy one of those? If you want to build your dream truck yourself, find one in a field somewhere that you can gut and build from scratch to your liking.
