By plexi,I'm assuming you mean acrylic... so you'd normally go 1/8" thicker on that size tank.
My usual "tank calculator" http://www.garf.org/Tank/BuildTank.asp says you could use 3/8" glass on that size tank.It also gives you 4 different ways to build that tank and provides a "cut list" for materials.
It'll also stop you if a dimension won't work.
Take the link and "play" with it .... might be fun.