What is the most fuel efficient engine I can replace a 1990 Chevrolet 350 with in my 4wd z71 stepside truck?

Need to replace engine AND transmission in my 1990 4wd chevrolet 71 stepside pick up truck. This is daily driver, never needing anymore towing power than necessary to pull a trailer full of mulch home to the garden. I generally want to reduce fuel costs as much as possible and must keep 4wd, but don't need speed or power beyond what a trailer of mulch requires. What engine and transmission combination do you recommend for this vehicle?