I have killed a pile of elk over the years three of them were with a muzzleloader. Elk are tough! Guys that have never actually shot them have no idea just how tough they are. I have seen them shot multiple times with Large cartridge guns and stay on their feet.
I personally use paper patched conical's. You're asking about PRB and I am going to admit that I have no personal knowledge of using PRB on elk. I have hunted with guys that have used them. Your idea to use a minimum of 54 cal is a good plan. The guys that have said to get close as you can is a solid plan. Shooting through the lungs and staying away from the shoulder is probably the best advise.
Using as stout of a load as you can is a good plan. That said too much powder could negatively effect accuracy. I would shoot the stoutest load that you can and keep good accuracy.
The most important info I can give is waiting for the right shot. I can't emphasize enough how important it is to get the right shot. If you call the elk in it will be quartering to you or straight on. Both of these shots are bad with a PRB. Waiting for the elk to turn and give you a side shot or a quartering away is the best bet.
Good luck on the hunt.