you could have it milled flat at a machine shop. One thing about your belt sander....some automotive machine shops use a giant belt sander mounted ridgid to a table. It is a special type that has a belt several feet wide. It is used to clean and flatten gasket surfaces on cast iron exhaust manifold and such. You could mount you belt sander in a vise upside down. Then hold the manifold to the moving belt to try and make it flat. It will take more material from one side than the other and that is normal, so the trick is to not try and rotate the flange around but just hold it one spot. It will make it flat, but maybe at a tiny angle. Will not hurt. If you try and rotate the flange around, you may find that you will have a high spot all around the middle. Try practicing on a like size chunk of metal. If it does not work, no harm done, just take it to a machine shop to be milled.
