There is no "gasket" between the valve body and the transmission case on a 904 or 727. There is just a steel separator plate that is part of the valve body. I think that is what you are referring to. No special process or precautions other than make sure it is clean and no big deep scratches or gouges where it sits on the case. Cleaning should NOT include any scraper, sanding, or other abrasive, just solvent and if necessary a rag or plastic bristle brush. If using a rag make sure no lint is left behind. Best to finish with a solvent and blow dry with compressed air.