Mustangs were not built to last decades, and some of the sheet metal parts were not properly protected with paint or preservative.
Especially problematic in early models (65/66/67) is cowl rust. The cowl is an enclosed box that allows air to pass from the grill in front of the windshield into the passenger compartment via vents under the dash. Water drains out underneath the car via tunnels at the corners of the cowl.
Unfortunately, this cowl was not painted, and the tunnels sometimes clog with dirt and debris, preventing water from draining properly. The cowl sheet metal then rusts away, and water leaks in under the dash.
Even more unfortunate is the construction method Ford used to create this cowl---it's welded shut. To get to it, you need to remove the fenders and hood, break open 130+ spot welds, cut out the rust and weld in new sheet metal.
There are repair kits that let you glue in plastic pieces from under the dash, but this is at best a temporary fix---rust continues to eat away the metal and eventually the glued-in pieces will fail.
Other areas where rust can be a major problem, and sometimes difficult to spot unless you peer closely:
Before you start looking at cars, you should decide what you want to do with your Mustang. Is it a daily driver? a show car? a restoration project?
Some owners (and judges in concours competitions) consider it important that Mustangs be as close to original condition as possible---meaning same engine, same color paint, same decor as it had when it left the factory.
They ascertain this by decoding the numbers on the car's data plate (inside driver's door) and comparing the VIN (vehicle identification number) on the data plate with the VIN stamped on the inner lip of the fenders on early Mustangs (visible when the hood is up on the driver's side; you have to take the fender off to see it on the passenger side) or riveted to the dash next to the windshield on later cars.
If the numbers don't match, it means the car has had a new door or fender, or possibly a new and incorrect data plate reproduction installed. Some models (Hi-Po Mustangs, and later model big blocks) also had the VIN, or a portion of it, stamped onto the engine block. Matching---or non-matching---numbers can affect the value of your car if you are buying from or selling to a collector.
However, if all you're interested in is a cool car that goes fast, these numbers may not be very important to you. They also don't count much in "people's choice" or "restomod" (restored with modifications) competitions.
One modification to be on the alert for, however, is the aftermarket shift from a 6-cylinder to a V8.
There's nothing wrong with this, if done right; but it does involve more than just swapping engines. The entire drive train, brakes and suspension have to be upgraded, as well. Look at the wheels, and count the number of lug nuts. Six cylinder cars had four lugs per wheel; if you count five, then the swap was probably done correctly.
submitted by a65ragtop, adapted from an exchange of messages on the Vintage Mustang Forum (which is also where the decoding page is located)