Hey Mike I restored a 65 Mustang convertible 2 years ago. It also had a leak in one cylinder and the lines were starting to look brittle. I was knew I was selling the car, so I replaced only the leaking cylinder and installed new line. If it was a car i was planning to keep I would have replaced both cylinders, and the line.
when you raise and lower the top you want both cylinders pulling their fair share, so getting both done would seem the way to go. When I removed the old line it had hairline cracks developing, and with how messy that hydraulic fluid can be, I would just rather do it all with the rear interior out, then go cheap and pay for it later. (make sure you properly bleed the lines when replacing a cylinder or installing new line)
Hope this helps.
