So after quite a bit of deliberation, I think I've decided to go with a traditional glass tank on this build.. I really wanted the size of the 400, but after labor, etc. was all added up, it would have been around $1500 for the tank. I can get a planet aquarium 310 for around $2700 delivered, with a crystal front, overflow, etc. This will give me a full viewing area, and more peace of mind from catastrophic leakage.
I'll, sadly, have to start a new build thread.
Fortunately, the 300 will be plenty big for a fully grown cowfish, with plenty of room for other tank inhabitants. The 400, while nice for the extra size, will be more expensive to maintain, and doesn't really add a lot of room for additional livestock comparatively.