I imagine if you're going to grow one 300 year old tree, then your best bet is obscurity. Find a stable very-rural area that's not prone to bushfires, plant one

I imagine if you're going to grow one 300 year old tree, then your best bet is obscurity. Find a stable very-rural area that's not prone to bushfires, plant one tree and make sure it's doing well for a few years, come back 300 years later, you're done. If you're not going the obscurity path then you'd really want to scale it up - there's not much difference between security for one tree and security for 100 trees.