William,

That'* good for an epoxy putty, but normal JB Weld says it can do 550 degrees and has a tensile strength of 3960psi.

Maybe some kind of high temperature silicone would handle the expansion/contraction better if the JB Weld fails?

Something like this: Hi-Temp Red Silicone | J-B Weld

I'd not use something silicone based, you want the Epoxy stuff that hardens when used for sealing a crack in a LIM.
What I posted a link to earlier is all I would have attempted to use.

I wouldn't even attempt to seal something like that. Thing with cracks is, they get bigger over time. Do it right the first time.

Of course Mike, totally agree, sealing a crack like that is only ever a temp fix.
The constant heating up, and cooling down cycles would definitely make it bigger over time.