I will be installing a 8 point cage in my 70 Dart in the next few months along with replacing ALL the interior including the headliner.
If you are going to do it at a later date you will be cutting into your rear seat also for the bars that go into the trunk area. The notches that will need to go thru your front and back seats along with your carpet can easily be done at the time you put the cage in. I would proceed and do it when needed. JMO