When Ford was building Mustangs some 40 years ago, the technologies that prevent corrosion in modern cars weren't available. We're sure those cars weren't designed to be around this long, surviving the salted roads, and hundreds of thousands of miles of body flex, suspension cycling, and more.
This is why just about every classic Mustang (or Ford, for that matter) needs some form of sheetmetal repair. One of the most common repairs is the floorpan, again, due to road salt, as well as lack of undercoating, and rotted cowl assemblies that allow water to migrate under the carpet and rust the floor from the inside out.
In the past, repairing the floor assembly entailed several repair sections installed individually. These are considered "short pans." Later, these pans were manufactured as larger repair panels and are commonly referred to as "long pans." These long pans allow easy replacement of the full floor on each side with a minimum of fuss. The downside to the long-pan assembly is that if there's any damage to the transmission tunnel (such as a modified shifter opening) or extensive rust that's above the new pan's installed area, you'll be making custom patches, not to mention the labor involved in cutting out and welding in all the various panels.
The sheetmetal wizards at Dynacorn have created a much better solution for '65-'68 Mustang floorpan repairs by introducing a one-piece floor assembly. As with the OE part, this one-piece floor is stamped out of one sheet of steel, and includes the reinforced shifter opening with mounting holes, preinstalled seatbelt stud plates, rear torque-box upper panels, and seat riser panels. Installing the one-piece floor is similar to the individual panel installation, and we feel it takes even less time since there's less welding to do and fewer welding materials used.
Follow along and see for yourself how easy the one-piece floor is to install and how much better the final result looks, as we replace the original floor in a '66 Mustang K-code fastback being restored/ restomodded at Classic Creations of Central Florida.
 |  |  There are multiple options...  There are multiple options when it comes to removing the old floor. |
 We used a reciprocating saw...  We used a reciprocating saw to cut the floor in sections. |  Because this particular fastback...  Because this particular fastback was undergoing a full restoration, we didn't have to worry about fuel or brake lines underneath or having to remove the interior. |  Make sure your framerails...  Make sure your framerails are solidly supported with jackstands before beginning the removal of the floor. |
 When cutting around the inner...  When cutting around the inner floor support, be careful not to cut off the end. |  You'll also have to drill...  You'll also have to drill out the spot welds between the floor and the inner floor support to separate the two. |  The use of a cutting wheel/grinder...  The use of a cutting wheel/grinder may be necessary for some tight areas when removing the old floor. |