Cracked exhaust mainfold

I have recently discovered a small crack in my exhaust manifold right at where the first 02 sensor is located. The crack is maybe 1/2" long traveling vertically up from the sensor. I know it`s only going to get bigger. I am considering using JB weld to try a quick fix.
Do you think this will work?
JB weld advertises it will hold up to a temp. Of 600 degrees F.
How hot does the exhaust manifold usually get?
Thanks.