Midwest is currently building me a 300 with 3/4" sides.
Chris said the back panel will need to be painted because black acrylic isn't available in 3/4" stock.
I'm looking for a 300-ish gallon tank as well and that's what I was told. I decided to pass because I didn't want to pay $2000+ for a tank with a painted background. I was able to find cast acrylic sheets in black or blue in the dimensions required online so I know they make them. I guess Midwest only uses Reynolds who doesn't make colored sheets that thick.
I asked Envision Acrylics if a tank that size with a colored back was possible and they said they could do it.