When cutting out a rusted trunk do you cut off the gas tank mounts or do you leave them on ? Mine are in good condition.If I leave them on how do I detach them from the floor?

When I repaired my trunk I left the supports. What I did was drill holes from the underside at the edges of the suports and then used a air hack-saw or you could use a sawzaw to section out the pieces. Once this is done I used a die grinder with a cutting wheel to remove the metal over the support.

Another possibility is to get one of those special drill bits that are for drilling factory spot welds and drill the supports and then just cut out the edges where you need to and remove.

Hope this helps
Mark