[LEEVON]

Here's a trick I learned for valve covers and oil pans. If you're willing to use a cork gasket. Get the pan surface as straight as possible, then use the "right stuff" gasket maker and apply to the gasket and pan until tacky. Then set them both, gasket side down on the flattest surface you have (I personally use a 5/16" steel welding table). Put something heavy on top and leave it overnight. The gasket maker is thick and will allow some variances while curing. Your gasket surface will be as flat as the surface you placed it on, same as an engine block or cylinder heads. Works REALLY well for thin valve covers.

OR, get straight as possible and see if you can find the thick rubber Felpro gasket, they are also forgiving. As for straightening, don't be afraid to use a heavy dead blow hammer to tweak the whole thing from the top, or flange pliers to move the edge, whatever it takes that won't score the gasket surface badly. It's just metal, make it your b**ch.