I just was dealing with this exact thing with my 180g. I read for 3 hrs all kinds of responses. The common theme was yes foam acrylic, no-foam glass. I read(don't ask me where) that a guy has been doing steel stands for 30+ yrs and always has gaps. The thing I learned was this: the corners/ends are the most important to support level. Level front to back and level side to side/end to end. The tank doesn't actually require center bracing as long as it is a solid glass bottom, meaning, a bottom pane with the sides front and back panes resting on the bottom glass pane. The trim should be completly covering all exposed edges.
Anyway, this is what I read. Go on a Google hunt and start reading.
I would say, by the way, a .5" gap is a very large gap. Maybe want to investigate a support in the middle with a sag like that.
Good luck.