I agree that you should just do some maintainence to the 360 and bolt it in.
As for mileage, this always comes down to someone with little money trying to stretch it a bit further.
The cheapest way to increase mileage in most cases is to buy a $300-700 MPG beater.
Yes you can low-buck together a low RPM high torque small block and tune it to get decent mileage. But you are still out the time, money and effort to push a heavy brick through the air.
Examples from ID
http://boise.craigslist.org/cto/2545556261.htmlhttp://boise.craigslist.org/cto/2545409750.html
