Diy 320 gallon plywood tank

Hi, I have been thinking of doing a large plywood tank. It will be 82"x36"x25" which comes up to 320 gallons. It will be a pretty cheap build considering that i already have the glass (74"x24"x1/2") and the pump (1000gph). I will use 3/4 inch plywood and dry-lok to waterproof it. It will house my three res turtles. However this build will not start until I finish up my 28g bow front build.
I was bored so i made a quick mockup in sketch up.

Well after doing a lot of research I have come up with a few ways to seal the tank.
I have decided that Epoxy, Pond Coat and liquid rubber are too expensive.
1. Drylok on plyood
2. Drylok on cement board
3. Pond Liner
4. Pvc sheet

I am kinda leaning towards number 2-drylok on cement board. I am certain that the drylok will stick and seal the cement board for a long time. I could also probably get away with 1/4" cement board in a 3/4" plywood box
Also I changed the design a bit, I made it 30" wide instead of 36" to be more door friendly
Final dimensions= 84''x30"x25" (270 gallons)

Is this stuff waterproof?
http://www.canadiantire.ca/AST/brow...769P/Solignum+Epoxy+Floor+Paint.jsp?locale=en