A little data and opinions to put in the decision making hopper.
I have owned a 2001 8.1 4 sp. 4.10 Suburban, and a 2009 6.0 6 sp. 3.73 Suburban. The 2009 gets 1 - 2 more MPG, in the neighborhood of 14 vs 12.
Given the choice I would still own the 2001, with my top criteria being driving pleasure.
Assuming the van’s use is mainly highway and the objective is reducing fuel use, I would change the the differential to a lower numerical value. Much simpler, cheaper, and fewer potential headaches then an engine swap. My guess is the payback for an engine swap would be many years down the road.
I have been told that the market for used 8.1s is poor, but have not verified that. At first this didn’t make sense to me, but the explanation was that most people looking to an engine swap are going the LS route which is where the aftermarket has focused, so the 8.1 doesn’t show up on their radar.