...density is the correct foam to use for seat cushions that are to be made to replace seat springs? Is there an industry standard (or rating) as far as density goes? Can someone 'in the know' recommend a number that I should ask for? The application for this seat would be in a speedster/modified/T-bucket type vehicle. Also, is there a different "firmness" needed for the back-rests or should they be made from the same type foam?