[tucsonjwt]

Having owned both 350 and 454 squares, I have an opinion on both. My 3 squares were all 3/4 ton trucks. In my experience, when you have a 3/4 ton truck or heavier, you won't get good gas mileage regardless of the engine, unless you have a diesel, which has its own issues.
It is true that parts for a 454 are more than a 350 and more difficult to find. However, standard repair parts are not much more. Unique engine parts can be a challenge, but if you have the time to search for used parts they are not more expensive. I have one quirk in my current 454 - I have had trouble finding a crank pulley (no longer sold by GM), so I finally purchased a used one on Ebay for $65. (My crank pulley somehow got a loose bolt and flew off, tearing it up - this has never happened to me before and probably never will again.)
The best engine/trans combo I have had is a 454/TH400, but I think a 454 with overdrive transmission (700R4 or newer) would be best.
Because 454s have a bad gas mileage reputation they are easy to find. They are not cheap because of hot rod and boat demand, but you can get a whole vehicle with a 454, take out the engine/trans and sell the rest for $$. I would not be surprised if you could get an old 454 vehicle, take out the engine, and sell the remaining parts for break even or better. A "free" 454 beats a built 383 any day in my book.
I don't think you will get more than 10-12 mpg whether your have a 350, 454, or 383.
How many miles will you drive this truck per year? Unless you drive a lot, a few mpg difference will not justify spending big $$ in a different engine.
I will say that a properly tuned 454 is a dream to drive. With a properly adjusted Qjet carb I have achieved 11-12 city mpg. 14 mpg on a rare highway trip. But predicting mpg is a guesswork at best. Good luck.