The biggest issue here is the cut wiring harness. You're going to have to replace the entire underhood harness and it will need to be from another van, because the ECU is under the hood on these. .

Vans were usually a couple years behind trucks on the upgrades or changes, and the Express is no exception, getting the LS motors and related gear with the 2003 refresh.

You won't notice much difference in the two engines, unless you live in a relatively flat area. Once you go to 4x4 you're going to be lucky to pull 12 out of it even free falling from a helicopter.