Something I been thinking about, and it might be worth a look. Federal is running 300 Ultra brass now, Norma might follow if we're lucky, but I doubt we'll see the 338 Ultra brass anytime soon.
Necking up the 300 to 338 would be a better way to go, unless you want to use the 338 Lapua case and be done with it. I won't argue that the 338/378 is a darn good performer, but I will say brass is nearly $2 each, and it's not going to last very long either.
A good barrel is what I'd start with, pick the cartridge and find a smith with the reamers that don't want to retire on your rifle. Select the action (I like the 700 or the 70) and stock and set back and wait for a bit. I bet you could get close to 3000 fps with the 300gr SMK and a 338/300 Ultra appropriately throated in a 32" bbl.
Up here, law says you can't use a 30 cal for Buffalo, although I know someone who killed one recently with a 300 Ultra and 180gr Scirocco through the shoulder and then exited, at 150 yards.
I know about 5 guys who killed Buff in the last couple weeks, most used the 40's and bigger though.
My personal choice at this point would be a 338 Lapua Imp for out to 2000 yards. Someday...