Well the pot I have is 30 gallons but I'm sitting it on top of my raised garden bed and I cut large holes in the bottom of the 30 so when it reaches the bottom it will be able to keep going just gonna bury the pot a few inches in the garden.
Ya, a big pot like that you could burry eight inches no prob. Id dig a big ass hole under it and replace the soil with good stuff. Usually a three deep and five-six foot wide hole will work.
You sould be able to pull 3 lb plants off that sized hole but id personally just drop the whole plant in there because the roots will spread better for you. The pot will restrain lateral growth alot. Keep it in the pot for a good while and then drop it in when its big enough to hold the soil together in that 30 gal.
Good soil planted straight in the ground in Oregon should build you a 10 high, and 10' wide plant no problem. Even taller and wider if you do the hole right and your starting pretty early too. Itll be big.