These trucks are as aerodynamic as a garage door. The taller they are, the worse mileage they will get.
That is just one issue.
Low compression, wrong cam... Yeah, I agree. I had that cam in a Motor home 440 and while it screamed at WOT, at part throttle it was a slug like yours. I may have averaged 8 mpg freeway (UNloaded) with the 4.10 gear and 32" tires.

If you are trying to keep from swapping engines, milled heads on that 400 can raise the CR by about a point. Add in a stock 440 Magnum cam with better flowing exhaust and it should greatly improve. It will never be a 20 mpg truck even with fuel injection, overdrive and bicycle width tires. What numbers are you looking to get? 12 ? 14?
I agree that your transmission is a POS for a truck not used for low speed grunt work. I have the same trans in my 75 Power Wagon. I'm swapping in a 727 since I'm not a fan of manual transmissions. If mine was a 318 or a 360, I'd seriously look into a 518. There is room there for it and the .70 lock up overdrive helps. When I had a Gear Vendors in my 70 Charger, even their .78 OD made a noticeable difference in cruise rpm.
Do you hammer the truck much or is it a mild daily driver? The 833 OD is not a bad trans behind mild big blocks if you treat it right. I'll bet it has lower rotating mass than your 435 so it saves HP there. The OD is near .70 so you win again. The gear spacing is okay from 2-3-4. The 1-2 is a steep drop off though.

Oh, NICE looking truck!