I just happen to be preparing to do a V8 swap on a 1999 AWD.
The engine of choice is a Vortec 5.7 VIN R engine, although a 5.0 engine would work also. As close to the year of your van is a good rule, but any Vortec 5.0 or 5.7 should work, you may have to make some changes on unlike years, I'm not sure.
You will need a custom oil pan, engine mounts,
http://www.jagsthatrun.com, a connector for the fuel injector, and to add two wires from the PCM to the new injector connector, and splice in two grounds for the same. You will have to get your PCM re-flashed to add the V8 trim. I'm getting my PCM flashed by the guy at
http://www.lt1swap.com and all of the wiring info is there as well.
Try to find a complete engine with harness so it will be easier to add the injector wires, although you will reuse your original V6 harness.
You will have to either clearance the frame, machine the manifold, or buy custom headers since the right side is tight. I plan to clearance the frame.
I'll be documenting mine as I do it, if I'm still around. My wife has figured out that something is up and that the new engine may not be a 4.3. #-o
I'm also building a 1989 Safari 4x4 with a TPI 350, SM465 4 speed, and a NP241C transfer. case.