saftey rating

So i got a free tank but i don't like the setup its 8' long 2.5' wide and 3' high. What i want to do is buy a 4'x8' and 2x 3'x4'of 5/8 glass so its final size would be 8'x4'x3' all 5/8 glass. The front and back would be tempered as that the old glass is. Tank 8'x4'x3' all 5/8 mix tempered and...