No sweat Squid! You are on the right path. Yes, I saw that on the rubbermaid, too. 25".
Since you are going to be doing bio and mechanical on the tank, you'll need a layer of mats first and then the strapping on top. I highly recomment Matala mats, and you will be able to get them in the size to cut to the stock tank and lay them on top of the light grid. They are a tad pricey, but they last for eons, so it's an investment. I would use a green and black matala laid one on top of the other, and then the strapping on top of that for bio. My best guess is that 1500 feet might be enough based on the fact you need mats/sponges down first.
PM DODad on the matala, he'll get a better price than what you see online. Give him the inside dimensions of the stock tank and see what fits in there in one piece or at least two halves. A half sheet of matala might be enough. to fit in. You want to cut it perfectly to fit the stock tank body inside so that the poop will stay down while the clean water rises. And matala has nice density to fit tightly in place.
I have a 2 1/2" hole saw bit that you can borrow to do your holes in the tank. Those darned things are pricey at Lowes!
Just go slowly and think everything through. Heights of things such as waterline and such is going to be important to match everything up properly. Height of your streambed in relation to your pond, height of stock tank on waterline, etc. I think with your streambed, you can just put more angle on it perhaps so it heads more steeply into the pond (raise it up a bit on the farthest part where your waterfall will help. You don't want that pond water backtracking into your streambed.
You've got a good scenario there to use gravity, without having to do a filter pit due to the way your property slopes away from the pond. Lucky you! If you go with a BD (retro or regular), you can take advantage of that graviity flow. Of course, you don't have to do a BD at all. You just need to decide what is best for your situation.
You also need to think about that run-off you will have from your neighbors yard that is higher than yours. You can do something with the corrugated pipe you mentioned, but thinking about it, I would also do something with short barrier to ensure water is going down your slope. Some kinda landscape edging that you can buy at lowes to ensure your neighbors run off doesn't come over the pond.
Oh, and I have that quart of liner prep liquid you can use when it comes time to doing your streambed seam. I've got more than I will ever need to use in a lifetime.
Keep digging!