The 4" QuadVan package already has more travel than a Quigley. Also, QuadVan uses foam jounce stops which are compressible, instead of re-using the stock rubber wedges like Quigley, which are a harder compound because of the travel ratio of the original twin-beam axle.
That said.... not knowing which dampers you have, the RIP/FOX dampers could be an upgrade, possibly a big upgrade if you don't have the dual-Bilstein setup. Also if your van has put on some weight since the conversion, higher rated front springs can be beneficial too. Those do not need to come from Agile. I bought mine through my local alignment shop.
2000 E450 dually V10 wagon