FWIW, I think I would do the front and back in 1.5" acrylic. I had my tank custom made over 20 years ago. The tank is 48"x96"x24" tall. I went with 1.5" front and rear to insure there was no bowing or additional stress on the viewing panes. 20 years later, there is no bow or crazing what so ever and the tank has held up wonderfully. With such a long panel, I think you should play it safe and go thicker. I also think the top should be 1.5" as the top of the tank will want to deflect and bow. Especially with the openings and over time, cracks that start at the corners of the openings can be an issue. Thicker is better.
I'm not a big fan of seams in the viewing panes and if it were me, I'd probably avoid it even if it meant reducing the size a bit and adding some drywall or wood work to fill in the gaps on the sides of the tank. That said, if your manufacturer is reputable when it comes to building large tanks then you should be fine.