Welcome aboard! If your gona mount them to a roll cage then you need the hoops or the bar behind the seats. Most cages dont have that bar there. If it mounts to the floor then it would need a longer strap than if it went to a cage bar id think.
Im not a fan of 4 or 5 point harnesses. Too much crap to deal with getting in and out of the seats. Might be nice if running the baja to keep you from hitting your head on the top but on a daily driver to much BS for me.
Put them in if its what you wnat to do but im guessing that after a few months youll wish you never bought them! There not as cool as you might think, even in a muscle car!
Im not exactly sure on this but i dont think you want them mounted to the cage unless the seats are mounted to the cage also. No sense in having 4 point harness if the cage or seat detach from the body. What im trying to say is if your gona get that crazy about safety then do it right and build your cage with the seat bars as well as the harness bars. Or you doing it just for looks?%)