Often it is easier to raise the vehicle, and access the plugs from underneath.
Whatever is in the way needs to be moved. From what I have read, there is, in some cases, a strut from the head, reaching up to support part of the intake system. Remove the bolt from the head, and loosen the bolt on the manifold, then swing the strut out of the way. In some cases, the alternator will need to be moved a bit, requiring removing the drive belt.
In any case, get yourself under the van, get a good light and a mirror or two, and scope out what's in the way.