We'll say this plainly: a new install is not a spring swap. If you're comfortable with basic tools, replacing a broken cable or a bent hinge on an existing door is a reasonable weekend job. Setting a new door into an opening that's been sitting since the early 1970s - squaring the jamb, resetting track brackets, balancing a spring to a panel weight that may be different from what came out - is not.
We've seen installs go bad the way most DIY installs go bad: a track bolted slightly off-plumb, a door that runs fine for a month and then starts binding as the house settles seasonally. On a home this age, the framing has already moved once or twice over fifty years, and it'll keep moving a little.