460" engine, 4:10 rear axle and 3 speed automatic transmission. 7-8 MPG would be hopeful. Driving 55 will really help, while 65 might see much closer to 7 mpg.
Give it a tune up, try to lean it out as much as possible and it might improve the mileage a bit. I replaced the distributor on my 75 F-350 with 460" and got a lot more power, and mileage improved too.
But there is only so much that can be done with a carb. If the floats have a leak in them, it will flood the engine with gas, causing 2 MPG until it is fixed, and it will only run 'decent' while the engine is cold, once warm it will not run worth beans. A simple carb rebuild kit will replace the leaking floats, and it will run great again. If it has been sitting around a long time, the floats can start to leak. Also old fuel is no good for the system. Hopefully it is not all that full, and you can use it up quickly.
Old brake fluid is also really bad. If more than 7 years old (the original owner might have never changed the brake fluid in 20+ years) the brake fluid should be bled out, and replaced with Ford heavy duty motorhome brake fluid. It is very cheap at the local Ford dealer, you will need about 1 quart.
Of course if the engine has not started in a while, you might want to change the oil before starting it. Oil can become very acidic while parked for more than 3-4 years. If they had used synthetic oil, it will have helped, and be fine to run now.
Good luck,
Fred.
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