The best increase in fuel mileage will be swapping in a overdrive transmission. I went from a c6 automatic to a zf in my truck, and gained 3mpg. Since you already have a manual, your increase may not be as much, but I would expect at least 2mpg.

I did also swap from a 302 to a 351w in another truck I had, and while I did not check the fuel mileage, it was very little if any worse than the 302. I have never had a six, so I don't know how it would do.

Getting the rpms down while cruising down the highway will give you a nice increase, and also make the truck quieter and it will have less wear on the engine. But you need to check your clutch linkage. If it's mechanical that will have to be swapped out, all the zf's were hydraulic clutch transmissions.

 

Or bolt a Gear Vender over drive to the back of the trany you have now, works with a push of a button.
Also Advance Adaptors has an over drive unit that bolts between the bell housing and the trany it uses a 2nd shift lever to work.
Dave ----

edit: my thinking on a smaller motor, don't!
You have a heavy truck now install a smaller motor it now has to work harder to do the same amount of work.
Lets just say you had the gas pedal down 1/4 of the way with the 351 now you install a 302 3/8 of the way down and say with the 300 1/2 way down, all to do the same amount of work.

 

I would put an efficient cam and timing set in it. I wish I had an F-250 with a 5.8

 

 

I am going to stick my neck out here. If I were doing it, it would probably take me 2 whole 8 hour days. That is $640, so I would figure $700 in labor.

 

 

OP has a 1982 F250.

ALL 1980/82 F100/350's have mechanical clutch linkage as do 1983 F100/350's except F250/350's with 6.9L diesel and 460 (these have a hydraulic clutch).

1984 was the first year that ALL F150/350's and Bronco's have a hydraulic clutch.

 

You would need a donor f250/350 pickup to put that engine in place. You would have the engine, but you don't have any of the parts required to install it, from the motor mounts to the bellhousing to fit. Different radiator, different wiring, etc, etc.