I am starting to replace the driver side floor pan and noticed that the raised ridge where the frame rail fits is not in the old pan, its on the passenger side but not the driver. As far as I can tell the pan is the original. Would it matter if I replaced the pan from about 3" in front of the front seat mount to the firewall with a section of 16 or 18 gauge steel without ridges or drain holes?

Structurally it won't matter.