Agree that 20 mpg is a realistic goal, I just don't think 25-30 is achievable.
292 would be great choice, or even a garden-variety 250. Not sure about fuel injection. Seems to me G.L. mentioned recently that GM trucks for export to Mexico (until recently) had 292s with throttle body injection. Aftermarket EFI is available, but not cheap at all.
Of course, with TBI you have the same problem that plagues carburetors on inline sixes - you can't get the fuel mixture exactly matched between the middle and end cylinders.
Another interesting possibility, if you can find one, would be the 3.0 L GM industrial/marine four. I'd say these would not be performance engines, but should pull well and get good fuel economy.
