Dave, the 4.3 would be fine for your application, but I don't think 25+ mpg is a realistic expectation, overdrive and efi notwithstanding. My '97 GMC Sonoma has the OD automatic and sequential fuel injection with a 2.2 four (Cavalier base engine), and the best I've ever seen is 27 mpg. You are talking about a heavier truck with poorer aerodynamics. Twenty might be achievable, though.
Another choice that might be a bit easier in a narrow engine compartment is the 3.4 60-degree V-6 from a Camaro. These are smooth and have great torque for their size and excellent fuel economy. You have to use the matching transmission, though as the fours and 2.8/3.1/3.4 V-6 family use a different bolt pattern than the 4.3, which uses the standard chevy V-8 pattern.
Good luck!
