Some things to keep in mind, what kind of silicone are you using? If it is GE I or II it is junk, I would not trust that much water with that stuff, I would get some quality silicone such as RTV 103 or 108. Also, silicone does not stick to acrylic which is why you would have leaks.
Just curious, what's the difference? Aren't they both 100% silicone?
Also I built a small (5g) tank shaped like an "N" out of nothing but acrylic and silicone and it's held together solid with no leaks for over a year. I know it's not the same as 425, but that one didn't have any bracing whatsoever. Literally acrylic and GE silicone.
I was pleased to find out last night while cleaning up the inside of this one, that the lexan is held in place with bolts instead of JUST silicone or adhesive of some kind. So it's good that at least the silicone I'm adding won't have to hold it in place, just stop water.