METAL ROOF OVER EXISTING METAL ROOF

I have a 1993 mobile home which has an old UNEVEN metal roof on it now. My question is, should I put 1X4 slates or use 2X4 s for the new metal roof? One contractor told me to use 2X4s every 24 inches apart . Three other contractors told me to use 1X4s. I think the existing mobile home has 2X2 s in the roof now. Also, should I have the new metal roof grounded? Thanks very much. Gloria