OK - let me rephrase that first question. I don't have the book that Paul mentioned, although I do have the photo Henri-Pierre posted and many others - all I could find online. Looking at them last night, some did have wheels that looked like those on the tractor, while others were less certain.
If I model this I'll be using the Wespe 1:35 model, and I know from experience that their stuff tends to be junk - I'm astounded that they just released a 1:16 version of this kit: why would someone want to pay almost a thousand dollars for a crap model, which no doubt uses the same design, just with larger ugly pieces? One major gripe I have is that the hood is a single casting, including poorly represented louvers and steering linkage. Elite Models produces a better version, but I can't see spending some 200 Euros for a kit and then using less than half of it for a project.
Anyway, the road wheels on all of their versions (see photo)don't look anything like those in any of the photos I've seen, so I guess that makes my real question, "Where can I find good quality replacement German wheels-tires that are similar, if not identical, to these?"

Jon - I'm far from an expert, but have to disagree for at least two reasons:
1) I don't know the exact date, but believe they didn't build any of these trucks with the external steering linkage after say 1940.
2) It's clearly a converted crane truck, with the railroad option, and I can't begin to imagine why the US Army would order a new truck with costly options that they had no intent of using.
"This also looks too 'clean' to be a field mod"
I don't know just what you mean by clean in this case, but we need to remember that the crane was removed (vs. major new stuff added), and to my eye the only thing I see added is possibly the headlights. Does anyone know if their style/mounting suggest that it was done during the war? My own take is that the low position wouldn't do much for illuminating the road, but does help keep them out of sight from the air - although, the beam would be another story.
Michael - I think the rear rail wheels are on the bottom level of the stowage rack with the spare tire on top, although I can only think that the US Army kept them there as ballast, vs. any intent to ever again tun it on a railroad.
As far as the front, that disk under the cab is indeed the mounting location for the rail wheel, but it's much too small in diameter to be the whole wheel itself. I'm guessing that the outer "rim" was stored and attached when needed. Notice the relative position of the bolt circle to that on this model.

This image also shows the stabilizer jacks extended, and I must confess the headlights have me wondering if those on the US truck might be original. This model was scratchbuilt by Jorge Lopez - I believe in 2000 - before there were any kits available. He calls it "the most impressive truck of the Second World War". No doubt fighting words for some of you ;-) Article at:
http://www.militarymodelling.com/news/article/faun-lk5-crane-truck/3427/I'm not a huge truck afficionado, but I found the original image while I was researching the LK5, and loved the long stretched-out look and the sense of graceful, understated power.
I recently tried contacting the survivor (merged) Faun company with no luck, and there's little info available (anyone have the references that Jorge mentions?), so I'm always happy to learn stuff from other LK5 fans.
As a parting "shot", here's one with a "boom car" serving the Austrian state railways in the 1970's - I believe it's parked in the Vienna main yard.

Charles