61 Dodge Lancer 6 to ? conversion

I have a customer who has a 61 Lancer two door. Clean car with 225 slant six and 904 push button auto. He lost a rod bearing in the slant and has asked me if it is possible to put a late model V-6 or ? in the car to improve fuel mileage and dependability. He was thinking something like a Dakota V-6 and auto. Has anyone ever done this type of swap and if so what are the pitfalls. I want him to let me fix the slant, but he didn't like the mileage. Any suggestions as to how the mileage can be improved?
HemimanDan