from the measurements shown it appears that you will be setting the side glasses on top of the bottom glass.
the measurements as shown for the front, back and bottom is perfect, however, the side glasses should be 12 inches less the glass thickness on each side as this glass will be sandwiched in between the front and the back glass. the top side of the side glass too needs to be cut less the glass thickness as this will make it a rimless tank. However, you can add braces to your tank on the inside bottom flushed to the sides by gluing glass strips about an inch wide and say 30 inches in length to the front and back glass to the bottom glass and also to the sides maybe 1 inch by 20 inches. This will make the tank take the water pressure and will work well as a rimless tank.
Most important will be the sealing with the silicon glue. See to it the you apply quite liberally and after attaching the panes together tie them together from the outside by means of some elastic strapping and keep it tied for a day.
the next day run a beading of glue on the insides and evenly apply them with a gloved finger for smoothness.
Most important to remember in a rimless glass the effect is best seen if the water is right till the top (spill level).
“Courageous, untroubled, mocking and violent-that is what Wisdom wants us to be. Wisdom is a woman, and loves only a warrior.”