Aw man. That's absolutely horrible. Have always enjoyed your posts about the van; after 18 years it's much more than just transport or a workhorse. Here's hoping someone has seen something, and it can be found before anything befalls it.
I would love to hear that the culprits have been apprehended and had a bentwood chair inserted into them through a variety of orifices, but that's probably just wishful thinking.
Insurance-wise, it's likely that the long-term hike in premiums would outstrip the value of any payout made, unless you have protected NCB (and even then maybe not). I made that mistake a few years ago by claiming on my home insurance for a sodding broken phone. But may be worth seeing if there are other, similar condition 305 vans for sale that you could show the insurer, should you want to go down that route and avoid a lowball offer.
+1 on ringing round any breakers or scrap metal dealers within the area, in case that's been the reason for the theft.