I'm thinking the transmission on the van currently is a 46rh and the newer one is 46re, yes?... the older van won't have the electronics to tell the electronically controlled transmission when to shift... would probably be easier to build the old transmission again rather than use the newer one. even with a few mild upgrades it'll be stronger.

I can understand the desire for the increase in power, but I would be shocked if even a well tuned 5.9 gets even as good on gas as the 5.2.

I have an even heavier van than yours (99 high top with a 5.2) and it does mid-upper teens on gas on the highway - really not too bad for a 7,000 pound "flying billboard" ;-). my 96 4x4 truck with a 5.9 in it is lucky to get 13 on the highway... not a direct comparison, but not the first vehicles I've had with 5.9 or 5.2 dodge engines in them.

the best "mild" upgrade might be to build your transmission with a transgo shift kit, higher pressures, better clutches, etc.. a LARGE cooler with no anti-drain valve and and decent (factory style) torque converter with little higher stall for some added power down low. Then if you want to do the engine swap later, you already have a transmission that you won't have to monkey with to make it work in your van.

Just a few thoughts to consider...

 

96 should be RE as well.

My 96 3/4 ton could pull 16-17 on the freeway, unloaded, with just my wife and I. On one trip, I even managed to average 18..... but, only once. (and that was a 3/4 ton 4x4, QC/SB, on 265's.)