First, welcome!!! This is a great wealth of knowlage here...
I have both a van, and a truck. The van is, as you said, a slug. The truck scoots. Even before I did some mods, it was quick.
The van has no intercooler, and is likely de-tuned to accomodate this... Mods to the air intake system on the van are much needed, and difficult at best.
The normal things, intake of some type, straight pipe/no cat the exhaust, gauges, and a DP tuner is what I have planed for my van. I will probly replace the muffler with a better one, the stock muffer is the biggest restriction in the exhaust. The stock pipe is ok, the muffer has to go. Walker BTM mufflers are popular, reasonable sound, good air flow.
Truck exhaust is close, the down pipe may be too long, and need cut, shortened, and re-welded. But, as I said, the stock pipe is ok for 400+ HP, its the muffler that's gotta go.
Fresh filters all the way around, synthetic fluids in the rearend, DP tuner w/ econo tune, that should do something for MPG and power.
'99 E350 DRW Stahl 11' util box van, 7.3 PSD, stock. 138k SOLD
'00 F350 extcab, 9' utility bed, dually 228k
E'99 F250 extcab, S/Bed 211k. Sidelined due to motor issues.
'00 Excursion Limited, 7.3PSD 294k
'84 Chevy K-30, 6.2L TD, woods/trail beater. 141k