Douglas,
Looks like a great project! If it were mine I think I'd stick to the 2 3/4" chamber if for no other reason than at some point someone (probably me) would be tempted to drop a full blown 3"NE in it and give it a go. If Marrikai is to be believed the shorter case is more than sufficient for buffalo and would be far more pleasant/managable on the butt end. Of course either way, as a single shot, you will have far more options regarding loads, BP, Nfb or NE. You may want to get 5-6 cases, trim them to 2 3/4 and give it a test run with various loads. I don't know about you but I'm getting to the age where getting the living snot kicked out of me is losing it's appeal, I'd rather enjoy the rifle than think "Oh crap, this is going to hurt" every time I take it to the range. I guess I'm not the man I used to be in some eyes but after forty years of shooting these rifles, it is what it is.
Take care!