2-part mold question (advice)

High Strength 3 refers to the tear strength - how it resists tearing and stretches a bit instead of tearing. I don't see why it wouldn't hold up fine as long as you have a solid, dedicated support (or dedicated mold box) on both the front and back of the wall. Personally, I'd do a two piece mold or one that is one piece with cuts halfway down the sides simply to make it easier to take the casting out.

Now since it's meant to not be single piece castings but part of a "wall", I would also probably go with a more "rigid" type of silicon like a mold putty simply because you don't have a lot of play in having to fit the pieces together. If you can, I'd go with a different type of silicon mold material since it has to fit other pieces. But if you can't, then make sure to have a mold box that is meant to hold only that mold all of the time, the HS 3 flexes so the mold box has to be rigid and the same shape every time you pour a cast or the finished casting won't fit together correctly.