There is no aftermarket piece. Your options are:
1. Do what that other person did.
2. Buy flanges and pipe and fabricate.
3. Find a friendly muffler shop,, take a manifold to them and ask what fits it, what parts are needed and if they will sell you the parts.
4. Have a friendly small muffler shop make you those two pieces.
5. Use the
internet<<<<<<click on this as an example.
6. Contact HOTRODJOHN
I my self would do no. 4 for sure. I believe you are correct about flat gasket needed at that connection. I have a crossover that can be cut to supply the connection sections. PM for further.