Yea JB weld isn't going to cut it for that......
Take the manifold completely off,remove the cat/o2 and take it to a welders shop.
I believe standard practice for welding cast (Im pretty sure) includes heating up the cast metal to a certain temperature then laying down the bead/filler.
If you need a welder lmk I know a stand up guy in TX that worst case you could ship it too.
After being properly welded properly it'll be stronger than factory.
At least that's how the 90+ volvo turbo manifolds are that I'm more familiar with.
Best of luck,cool car.