Glass-silicone-glass bonding (tension vs shear strength?)

I'm attempting to repair a glass aquarium.
Here is a drawing I made when I ordered it originally,

5524681760_00ba62f7fd_z.jpg


the top brace has developed 2 small cracks.
6894713566_4663da8c3d.jpg


So I'd like to empty the tank, then remove the old brace, and replace it.
But I'm trying to figure out what dimensions I need the new piece to be.

Can it be cut to match the outer dimensions of the vertical sides, and sit on top?
6910878064_9090b3788b.jpg


Or does it need to sit inside, flush with the top edge of the vertical sides?
7056962347_b6f034a323.jpg


The first style would be a lot easier to do in this situation, but as it sits, the brace is in the second spot.

Why can't I find anything about the differences between shear and tensile strength of this joint? Does it even matter? I can see that the vertical edges are experiencing both of these loads.

edit: I found this
http://www.mgchemicals.com/products/108.html [Broken]
Cured Properties - MECHANICAL
Hardness 30 (Shore A)
Tensile Strength 28 kg/cm2 (400 psi)
Elongation 450%
Tear Strength 8 kg/cm (45 lb/in)
Peel Strength 7 kg/cm (40 lb/in)
 
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