Personally I would replace line regardless of how it looks on the outside (if nothing else just based on the age of the car).

Heres my reasoning.... conventional brake fluid absorbs moisture, which eventually settles in the low spots in the system. While this is normally the wheel cylinders (as evidenced by the pits which are caused by rust that you often find in old wheel cylinders), the water can also be trapped in low spots/dips in the brake lines.

In 40 years, I have only seen 2 ruptures in steel brake lines, but in both cases the outside of the line looked fine, but it had rotted from the inside. While it is a dirty rotten job, and MAY not be necessary, it;s not a chance I would take especially on a single
reservoir brake system.

Just my 2 cents.