The biggest issue if the height of it as well as the quality of silcon and workmanship when making a rimmless tank. Standard factory tanks are built cheaply assuming that the rim will hold it together, but rimless are not. what were the dimensions of the 400l tank you had? The height was the same? 3mm difference in glass thickness is a lot.
A 120X60X45cm tank can be built in 8mm glass (not extremely safe but it has been done)
A 120X60X55 build in with the same glass will fail!
A 120X60X50cm tank can be built in 10mm glass
A 120X60X55cm tank with the same glass will fail!
A 120X60X60cm tank can be built in 12mm glass
A 120X60X65cm tank with the same glass willl fail!
Height not lenght is a big factor!
Most manufactors state the braces on the bottom help in holding the bottom seam together but it does nothing to prevent the top side seams from coming apart with the accidental pressure of cleaning algae too hard, or poor silcon and/or workmanship. Your better of to atleast put small 45 degree triangle corner brace on the top of the of the tank (like eurobraces, but only in the corners, to protect the seams, though will do nothing for center bowing).