I would stay with the 440, just go with an rv cam, vac secondary carb.
Lifting the truck & big tires will shitcan your mileage also.

No matter what you do, gas mileage will be bad.

My stock 89 w250 auto with 4:11's will get 12mpg, the stock 76 d300 dualie dump with a 400 2 barrel carb, np435 and 4:56's gets 8mpg.


Live with it........


Ed