Dave,
That would make the van even higher. I have 270mm under the axle and the 250mm castors have an overall height of 305mm before you try and make a mount.
I think if you made a pair of cradles up for under the axle with a 100mm castor on each corner this would offer the weight capacity and stability required.
I have used white nylon 80mm castors for moving my machinery around the garage. You have to be careful when ordering them as some give the weight limit for a set of 4 and others for individual castors. One of the machines I moved was 400+kg. The castors worked well but unfortunately my painted concrete floor did not fair well. The pressure through the 4 small contact areas was so great that it cracked and lifted the paint.

Colin

Early this year we started discussing these. "Hard" wheels for winter storage.
Randa