The bottom piece of glass should never be allowed to lay flat on any surface. It needs to be able to bow under the weight of the water. The edges of the sides and the bottom will not be fitted with each other. There bottom should be at least 1/4 inch up from the bottom of the sides. With the dimensions you want to use I would make it 1/2 inch up.
The top pieces of glass are strips of glass used to reinforce the tank so it doesn't bow really bad. The strips serve the same purpose of the plastic stirp on lfs
tanks that help keep the glass from bowing. With the length of your tank, I would put a 2 inch wide piece in the middle from front to back to help with this. It would basically look like this. Looking down on the top, you will see 4 2 inch wide pieces of glass, or whatever Garf recommends for the width. These pieces will go all the way around the tank on top and the sides. The ends should have the short pieces and the front and back should have a thin piece that goes all the way across the length and connects with the sides. These pieces are cut to fit acroos the top of the tank with no overhang.
Think of it like this, Take a cardboard box and cut the top flaps to 2 inches wide, this will give an idea of what it will look like with no overlapping. Take the short ones and cut out the 2 inches the front and back will overlap them. I don't know how better to explain it so if you have more quesitons please sebmit them and I'll try to help more.
If I might suggest, when I build my tank, I will be using slotted wood for the bottom to support the glass and the bottom and L cut wood around all the sides and the top. The slotted wood will be slotted to 1/2 inch deep to hold the sides and support the bottom at the edges.
Good luck, let me know if it works.