First, you want to make sure the roots are moist/wet right now. That's a very big transplant and the possibility of shock and/or plant death is significant. You also do not want to remove any more root structure as that will make survival chances worse. Trying to chip down into rock -- besides being a lot of work -- means that the bamboo will have a lessened opportunity to get nutrients and get rooted. If you must plant it in that location, definitely plant it in a large mound...and for a bamboo that size, I would aim for a mound at least 10 feet in diameter. After you get it planted, make sure to stabilize it with three stakes and some twine/rope for several months. Otherwise the wind will continue to blow it over. The advantage of planting in a mound is that it will be easier to rhizome prune and keep the bamboo contained to where you want it in the future. I've found that mounded bamboo rhizomes tend to be closer to the surface, and break the ground closer to the main plant.

Good luck, and don't be surprised if you have a lot of leaf drooping and/or loss. It's always difficult to transplant a bamboo which is that large.