Just a note the ones in your link Gizmo are not Fallshaw. The original pictures of them on the first page are, but in the specs they
are not.
Bit of a catch. As you said, it's very important that they are Fallshaw with 20mm high speed sealed bearings to stand up to the pressure but more importantly the quality of the plastic for the heat.
"Landyacht has tested most of them to destruction"

You need "Fallshaw" 400 x 8 KNO rim and tyre, with 2 PWB10 black adapters, to take 2 of 6004 rubber seal (20mm), high speed bearings. All times 2.

I get mine from,
http://staging.easyrollmh.com.au/wheels-castors/pneumatics/plastic-pneumatics/The bottom of the page for rim/tyre, across for bush PWB10 and with the second line down of the bearings 107mm x 20mm.
Hope this helps and we look forward to your build pics. Yell out if you get stuck.
You can also get Fallshaw steering head bushes for 20mm shaft. "SB 20" is the part number (about $1.70 each from memory)
They fit into exhaust tube "sn" said. ( I have just machined up heavy walled water pipe, for them to fit, as that's what I had on hand).
Grey are SB 3/4", black are SB 20mm ID, Inside diameter.
Your going to just love these LLMini's