If you have a 1970's Dodge Dart, chances are that you have some serious frame rail rot unless you live in the desert and your car never saw moisture.  The drivers side front frame rail is the first to go.  There are several choices for repairing these.  You can build up the area with new steel, you could get replacement pieces from a donor car, or buy new parts.  Fortunately, a company called Auto Rust Technicians has come to the rescue.  They produce frame rail cover caps for all the sections of your Dart Swinger for easy welding over top of the old ones.  These products are the way to go and I highly recommend them for replacement parts.  Alternatively, if you have access to a sheet metal bender, you can custom make these from 1/8 inch sheet metal to suit your needs.