aspear, i think your tank will be great for your cats. i think the best option for seeling your tank is to use abs sheets and seel them with abs cement and some silicone over the top. this will be long lasting, durable, look much better than liner, have no trouble with spines, be easier to work with, and be cheaper.
as far as your frame goes, i don't like it

the "rib" around the side doesn't do anything, and if it is continuous, it weekens the pond. i'm assuming you would like to insulate your tank from your cold basement floor, so bulid your walls, as well as the base, just like a studded wall in a house, but on 1 foot centers, or even less if you need piece of mind. than use standard insulation or foam board insulation if you think it may get wet.
then "ring" at least the top with some 2x4 for added strength. you won't need to use more than 2x4 and 3/4 plywood.
here are a couple cad pics of tanks i designed, a 300 and 1400 gallon pond, and a pic of my 300 gallon tank i built the same way i described to you.
in the pic of the real tank you can see the ring i'm talking about, also in one of the drawings, the black line you see is the ring.
any questions? did i miss anything?