Those dimensions seem very familiar.
![#wink](images/smilies/icon_wink.gif)
With 18" behind the tank I suggest you don't put on any weight.
![#lol](images/smilies/icon_lol.gif)
However perhaps you are also planning sump access from all sides which would resolve that issue.
If doing it again I also wouldn't waste money on starphire glass. Just one tiny grain of sand in the magfloat can leave a lot of scars.