first, there's no reason to restrict your search to vans, as all of these engines (in a given year/time period) were teh same across the board of body styles.
that said, you have to ask whats really improved, a 99 i worked on blew a plug, an 03 i worked on had 2 plugs break off in the head (infamous 2-piece design), required $150 of special tool to extract them, and over $20 per plug to get champion's one-piece design to correct the poor design.
IMHO, neither head is really a good design. i'm not exactly sure about 01 being the year, but its at least close. you might be able to study a parts-store website to see if you can find when they changed spark plug styles