There's an offline archive for download at
https://mega.nz/#F!iLwgXKRb!XvA-cUt5-8XyRPlgoVH-FQ
yw. In case you should wonder like I did when trying it the first time:
You'd have to download & extract the latest full archive (extractQ_1115.zip), after that download & extract the latest update (extractQ_diff.zip) to have all posts & images until 02/26.
Well json … You mean the original 8kun json? … or some custom json of your own design?
As a matter of fact, it would probably cost me (or any other script kiddy) half an hour or so to reformat the html into json.
Thing is, then the browser won't show the posts as beautifully anymore (i.e. incl. images, movies etc).
Let me know in case I can help with some simple script to get you the json you'd like.