MMike is suggesting you use gypsum board on your shower walls only
if you use his suggested waterproofing method, i.e., the Kerdi Shower System.
No one here will recommend you tile directly to any such board, including "green board" in any wet area.
You say you've "...installed a 34" x 48" shower pan and..." Does that mean some sort of pre-made receptor that's finished, a pre-made receptor that requires tile, or a traditional mud/liner/mud shower pan? Whatcha got?
Homer can get you 4x8 sheets of Hardibacker, by the way. But that stuff is not easy to handle in that size.
My opinion; worth price charged.