That's a toughy
Personally, I think if you can install a 6 point cage where the main hoop is kept far enough away from your head so it doesn't contact it in an accident it's ok. This is hard to do in a foxbody due to how tight the interior is (and how tall you are), so the main hoop would have to be pretty far behind you which requires it to lean back quite a bit since your bottom post mount can only go back so far. I also think it would be surprising to people to see how much you move around in an accident (especially a rollover) and how much room you would need to give. Hit your head hard enough on a roll bar (with or without padding) without a helmet and you're pretty much dead. Belts are made to stretch (both 3 point and 5 point harnesses) otherwise they would remove limbs or cut you in an hard enough accident. Lot's of variables to consider so I don't think there is one easy answer. If I were building a purely street mustang I would stick to proper through floor subframes, matrix bracing, and possibly an additional rocker panel bar run inside of the existing rocker panel that mounts to a vertical bar by your feet that ties into the upper cowl. Then some additional foot protection bracing from vertical bar to the footwell area. Past that, another bar from that vertical bar that ties into the strut mount. This woudl give you side and foot protection, as well as stiffen the core of the vehicle while not putting any bars by your head.
I hit my head on a rollbar once, in a full 5 point harness with helmet on, and it rocked my world. It had padding as well.
