Keniff, IIRC, I think the '04 vans were DBC not DBW. A better route to go IMO.
Less wiring and less problems, cheaper to repair down the road.

Pavetim,
From what I've gathered here and on other LS1 boards, the 4L60e/65e
probably isn't a good match for a 6.0 unless you plan on leaving it stock.
A moderately modded 6.0 will really tax a '60e and a heavily modded 6.0
will shred one pretty quick. GM put the 4L80e behind the 6.0 for a reason.
Save yourself the grief of re-programing the van ECM to run the 4L65e and
changing it back again when the '65e bites the dust.
Best to go 4L80e from the start.
The van ECM and harness should already support the '80e as is.

On another note, I've often thought the OBDll trucks (your 98) would be
easier to swap than the OBDl's, as alot of your truck's already setup to run
through the ECM to begin with. (VSS, cruise, TCC, ect.)
Somebody correct me if I'm wrong.........I'm new here!