I have a 120 which I am currently doing this. It is the same size as your tank but it goes back 24" instead of 18".
Problem #1 The older tanks did indeed use 1/2" glass but only on the 4ft panels. Your side panels (if your tank is like mine which it should be) is actually only 3/8". My glass shop is cutting two new sides for me in 1/2" glass for $65 a side (24"x24"). To rebuild you are going to need to take it apart so you will lose the bottom rim and will need to run foam under the tank on a flat surface. I plan to place it on a slab of finished granite with stainless trim covering the bottom and foam.
Problem #2 The glass you will be exposing by removing the rim is not finished and quite jagged. I was quoted by my glass shop $30-40 to have all four edges of all five panes polished/straighted with 45 degree edge beveling.
Problem #3 ADA uses 1/2" glass for their rimless 120 gallon tank without any euro-bracing. They also use a low-iron glass which is not as strong as standard glass. This would lead me to believe that you would be safe with no Eurobracing. The truth is, ADA has amazing craftsmanship and your glass will not be that straight on the edges. Truth is, for a 4ft pane, 1/16" tolerance is the standard. This is a little over 1.5mm allowing up to 1.5mm of silicone to be between the two sheets of glass, lowering the necessary burst pressure. At bare minimum, you need to add a lower euro-brace. I was quoted $50 from my local glass shop to have 2" eurobraces cut which I do intend to run on the bottom rather than the top.
Good luck!
