Best diesel engine?

[jhiggy94]

I am trying to look into an engine swap into my 72 k20. Right now it still has the original 350 in it. I am looking into the best engine for my needs. The main thing i want is to have decent gas mileage with the older truck to make it into a possible better daily driver/hauler. I know with a diesel i can hope for mid 20's mpg? But i also know that a diesel will be very expensive. I am just trying to get the best bang for my buck. Any opinions would be greatly appreciated.