I agree on using a racing seat and harness if you have a roll bar/cage. Anything lees than that (stock seats and lap belts bar or no bar) and you are at the mercy of a higher power in the case of an accident. The crash tests from the 60-60s era were brutal. One of the reasons I like beating on my cars at the track. Chances of a hard, head on front end collision are much lower with no telephone poles etc. At that point you end up abiding by the rules of where you race and add safety precautions for the street use.

A few things also come into play with design of the cage too. You don't want to be under the hoop. I have seen taller guys do this when laying the seat back or using a factory bench. It is acheived with a kicked out horizonal bar in the hoop instead of running it straight across so it does it's job. Bad place to be without a helmet, padding or not.

The other problem I see with the factory flip up seats of 67 and back is that they do not lock in position. Then you see the harness bolted to the floor behind the seat just right so in an accident the seat will fold you up as both it and you torso get intiment eith the dash. My 66 b bodies both have race seats because of this.

The rules say the hoop must be higher than you helmet so definitely check that as you put it together. For sure use swing out bars unless you are really throwing the world at it lol. I also like forward bars in our unibody mopars. Cab turned me on to the idea of running them outside of the fender apron. Way easier to work on the car with the bars out of the way.