I have a 70 Challenger convertible that has the 6 way seat. It doesn’t have the under floor pan brackets, they must have got lost when the previous owner restored it. Getting the repros has been on my list of things to do but I never have got to it. In the meantime I’ve owned the car over 20 years and put close to 10,000 miles on it, I weigh over 200# and I’ve never seen any evidence the floor or mounting has been damaged not having them. And I had the interior and carpet pulled out a few years ago to replace the carpet so did get to look at that area closely then.
You might want to see if you can find someone with a hardtop with 6 way seats and try sitting in it before you add the option. I’ve heard a number of E body owners complaining about it, as the mechanism raises the seat height and affects head room. I’m tall and I’ve never had a problem with mine, but 98% of the time when I drive my car the top is down, so headroom is never a factor! I think even top up, convertibles have more head room due to no headliner etc, as I’ve never felt uncomfortable the rare times I drive further than moving the car out of the garage top up either.