How would 8x32 tiles look with one vertical seam on shower walls that measure 39.5“ by 35.5” by 39.5"?
You don't.  I have installed around 50 showers and designed maybe 5 times that.  You don't buy tile that is just a little shorter than your width.  If you did buy this tile I would run it vertically.

Also I would think if you could possibly extend the edge walls as far as the tile.  You have to think of your shower as a big puzzle.  You want to match the pieces to your shower.  Larger pieces in a small shower are just odd.  It only works well if the pieces match up somehow and in your case they don't.  You are going to spend a lot of time staggering these things to look well.

Really the only thing you could do is a diamond pattern with something that % of the width and that is a really hard install.  Good luck - (buy something that works better).