There are always methods on older vehicles to get better mileage, but there are always trade-offs. Putting a V8 in it will not guarantee you better mileage. An old Suburban is like pushing a very heavy brick down the road and you will never see Civic like mileage.
Define exactly what you want it to do when you are done with it and the configuration of what you have now and there is a good possiblity you will receive several reasonable ideas.
