Looks like you can make a tank that is 350x300x300.
basically use the larges pieces for the front and back. The 350x300 is ready to go, but the other 350x350 would have to be trimmed back so that it is the same dimensions. Then, for the sides, the 300x300 is ready to go, but the 350x320 would also have to be trimmed back. You would be left not using the 300x270 at all (unless you want a part lid).
You can by a glass scorer from a hardware store. Basically you just score the glass where you want it to be cut using the help of ruler or something else that is straight. Then you apply pressure to the piece against the corner of a table or use some glass pliers to 'bite' off pieces and the glass should split right along the line . I've done this with smaller pieces of glass and tiles but I don't know how hard it would be on a large piece or what type of glass this is. I'm sure there are vids on youtube showing this process.
HOWEVER, the glass edges will be really sharp...
I would recommend taking the pieces to a glass repair shop where they can cut it to the dimensions you need and also bevel the edges so you don't cut yourself!