Can I échange a 350 5.7 engine into my 305 engine on my van?

Hello, sure - anything is possible. It certainly would fit. The problem would be: engine management, wiring harnesses and electrical -- as well as how the components interface with the automatic transmission, related controls for that and the various other modules and controls on the vehicle. Since the 1997 2500 van had an option for a 5.7 from the factory, you could possibly piece together all the parts needed from a salvaged vehicle. It would take a significant effort, but anything is possible with some resources and know how. I guess the question would be: why bother? What are you trying to accomplish? And, what kind of investment of time and money into this could be better put into finding a vehicle that is already of your liking?