paintrider

My 4.3 V6 is getting tired and I would like some advice on which engine would be a good choice for a swap. I pull a 2 horse trailer with my truck, and trying to stay at 60 mph on the interstate and it keeps going into overdrive, putting the tach in the redline. I don't need a lot of muscle but I am seeking an alternative to the 4.3 Vortec. Any proven economical engine swaps that gets better mpg than 14 or so?