Ron
I bought a slightly used (4,000 miles) 1965 HiPo Mustang in December 1965. The biggest weakness in the drivetrain was the U-joints. I have no idea how many of them that I broke over the years. The HiPo had a toploader and a 9" rear end so the rest was quite strong. The engine could take a beating like nothing I have ever seen and it always came back looking for more. Mine had 4 piston disk brakes in the front so it actually stopped quite well for a mid 60s car. The big problem was that the pistons would stick. I sold that car for $1,500 2 or 3 years after I got married. A numbers matching "K" code is worth a bit more than that today.
Of course the early Mustangs were little more than a Falcon with sporty sheet metal and bucket seats, when you come right down to it. I liked those early Mustangs so much that I bought a '66 289 coupe, 6 years ago. I completely restored it and I drive it as much as I can. It has electroic ignition, poly urethane bushings everywhere, and SSBC disc brakes so it has been modernized just a little bit.
Wayne