Diesel Swap questions (Gas mileage, not power)

[BiggSixx1970]

Hello everyone!

I have a 1970 c20 250 I6, sm465 4spd w/granny gear, Eaton full floating rear end, HD rear coil springs. (will post pics when I get them uploaded)

I am looking into a diesel swap this upcoming spring, I am wanting to do this for gas mileage, not to roll coal and blow black smoke everywhere. A lot of people knock the 6.2 Detroit and say its weak and useless, however these people who make the comment are diesel guys that push their engines to the limits, I hear the block isn't as strong but as long as you aren't trying to build a hot-rodder they're solid engines. I have also considered a cummins 4bt (I have my reasons for choosing the 4bt over the 6bt, mostly weight and I just don't need that much power)

I have heard that with the right combination of diesel engine, transmission, and rear end gearing that you can get 20+ mpg, I have even heard guys with 4bt's getting close to 30mpg.

If I were going to swap my 250 for a 6.2 Detroit, what would I need to do to the frame of my pickup? do I need to beef anything up? it is a 3/4 ton after all. I have heard my transmission and rear end are all but indestructible.

What would be suitable for a transmission and rear end gearing to maximize gas mileage?

You may be asking what I intend to do with the 250 I6.. I plan on putting it on a stand and building it up for a turbo and eventually putting it back in when I get the truck closer to restored original condition. I got one with a 250 thinking it would get decent MPG.. turns out I was horribly mistaken, it barely gets over 12mpg on a good day with a fresh tune up, most of that is due to the low farm gears

This pickup is my daily driver so I need any ideas ya'll have to boost my MPG to the max. I live in a very green town in Kansas and we have pumps for bio diesel, and most of the fast food joints will give people they're used oil, so I may or may not burn vegetable oil in the future if I do this diesel swap

Thank you for the advice!