This is kinda the long way around the explanation but bear with me.
The reason the factory points are best is because they are reinforced. there are 2 pieces of metal sandwiched together which makes them stronger, like plywood. 16ga metal by itself is strong enough to keep its shape and bear the weight of a seat plus person, but when you add the dynamics of a 150-200 lb body on a seat acting as a lever then accelerate, decelerate and turn in both directions you are now adding additional stress. Over time it would have the same effect as if you took a long screwdriver through the hole and simply hogged it out or buckled the metal. The factory decided that multiple layers of thin metal can do the job of thicker and heavier metal in certain places like seat mount reinforcements. Most of the time the simple steel sandwich works fine.
As far as your idea goes, I wouldn't make a 16ga adapter from the new seats to the old holes. Like I said earlier 16ga won't hold the lateral stress from driving. If you used it to double up your floor pan and punch new holes that would be better since you'd have the plywood effect. But if you want true bolt in adapters with no new holes you'd be better off using thicker metal such as angle iron or heavy plate steel since the seat bolts will be hanging in space compared to the bolts through the floor. That creates a longer lever and more stress without support.
In the past I've put Challenger seats in a 77 Power Wagon, and I've made seat adapters for my 72 CJ. Both times I used 1/4 inch plate or angle iron simply because I couldn't get the sandwich effect for what I was trying to do. After testing (and driving the wheels off) the Power Wagon the 1/4 inch plate held up with minimal bending. What you're trying to do is easily possible it just needs to be thought through and designed properly so you don't end up in the back seat when you hit the loud pedal.
