65 mustang coupe

[Ron1]

You may want to run water into the cowl area and see if the water gets inside. It would drip down on where your feet normally are. Run your hands along the bottoms of the doors, and look for rust holes. Open the doors and see if there is any up-down movement when you lift up on the door handle. Get under the car and look for rust spots, to see if any standing water has ever been in the footwells. Check in the trunk, on the left and right side. There are some wells there where water would collect and rust thru.
Lastly make sure to look under the battery, to make sure the inner apron is still intact. Usually battery acid over the years will eat thru the apron and sometimes thru the frame rails, especially under one side of the radiator support.
If you are thinking about someday replacing the six with a V8, keep in mind that the 6 cylinder was built "cheap", so almost every thing would have to be replaced. Suspension, cooling, brakes, wheels (4 lug vs. 5 lug), drivetrain, everything and that is BIG bucks.

Ron