Well, I for one, didn't respond because my van has barn doors, so I can put 20" wide rear wheels on my van and the side doors will still open. LOL
But you're right. The slider might have clearance issues with a wide rear wheel and tire setup (I don't know at what point though).
The solution has been to get an extended arm kit (or whatever it is), but from what I've read, they're impossible to find, so people make their own.
I don't know how, or what the design requirements would be, as the only slider I've ever had was on a '70 Westfalia, and 5.5" wheels pose no threat to door clearance.
Several guys around here know all about this clearance issue and how to remedy it, and there may be some threads on it around here somewhere.
About the lifting of the rear of your van...
I don't know if it needs to be lifted for clearance, but it looks like it might need to be for stance. The front looks higher than the rear in the picture.