I think it'll be fine.
People like to refer to charts that have height as the ONLY factor in determining the thickness of glass/acrylic being used, but in reality, its much more complicated than that, but for common aquarium sizes, its a pretty safe rule to follow.
For such a narrow tank, I think the 1/2" acrylic will be fine. I don't think it was a reptile tank, because who in their right mind would use Acyrlic as a reptile tank. It would be scratched to hell.
I do think you will need a brace at the middle of the top. I, personally, would just put a clamp in the middle on the top, put the tank on some plywood on a level surface in my garage, and fill it up for a few weeks and check to make sure everything is fine.
Interesting looking tank. I always like seeing tanks that are unusual
.
If you're still nervous about the tank, it would make one hell of a paludarium setup!