2.5 instead of 4.3

Has anyone ever tried to swap out a 2.5 where a 4.3 was. Can I change the wire harness as a plug in ( take out the 4.3 harness ) and plug and plumb the a 2.5 in it's place . I have been driving an E250 4x4 van and the gas mileage has been way to costly . Want to stay with the 4x4 van , but increase gas mileage . Feed aback and comments welcomed

Thanks
Ric