so first I found a piece of granite that was long enfough to slide the whole manifold on it with keeping it flat.So tell us how you did it!
Next I used a razor blade and glass cleaner to clean the granite to make sure there is no dirt and debris on it.
Scraping the Granite while keeping the blade flat not to gouge into the granite.
After that I checked for flatness using my precision strait edge for checking engine blocks with my feeler gauge set. .0015 inch
I figured out the material on the manifold is really hard steel.
Used thin coat of spray glue on the back of the sand a paper sheets.
I was able to get 3 sheets glued next to each other in a row.
I placed the manifold on top only using the weight of the manifold I gently moved the manifold back and forth to remove material slowly holding the manifold as flat as possible by hand.
Occasionally checking the manifold with strait edge.
Checking 3 places accross and diagonally corner to corner in a X pattern making sure the manifold stays flat making I mark the manifolds high spots with a sharpie to be sure to remove the high spots.
Continue to remove material with 60-grit. Once things were getting close. Which was when the sand paper was loosing its cutting edge. I switch to 80-grit then to finish off to 120 gritt.
Sand paper clogged up quickly so I kept a shop vacuum around to remove the loose material.
I ended up using 3 sheets of sandpaper for each grit.
Total time 4hours
Total cost $15 CAD
Labour to do myself and a skill to practice priceless.