6" is pretty wide for a Type 3. There's a squareback on here with 18x8" rims I think, but you have to get the offset right. Any wider than that and you will start hitting stuff, like shock absorbers, bodywork, trailing arms and so forth. I can't see why you would bother tubbing a Type 3, 205mm tyres will give you all the grip you need and I don't think they would look very good on huge steamroller tyres. The suspension design would probably be quite a headache, and if you went too wide then the engine will get in the way, as it's sitting right between the rear wheels. You'd be much better off putting flared wheelarches on and fitting the extra wheel width outwards rather than inwards.
Here's the link to the biggest-wheeled Type 3 I've seen.
http://aussieveedubbers.com/viewtopic.php?tid=76445&page=1