RamThunder,
They are all aluminum. I like that because I have seen a number of the cost effective heavy steel boards that suffer rust too quickly, sometimes where you don't see it under the rubber anti-slip step pads or near poorly painted welds under the board. Not a huge deal, because I was going to repaint poorly painted welds if I went with steel boards as a just in case. But anyway, these ones have what I would characterize as a slight but normal flex under my 210 lbs. I consider them to be plenty strong. There is also flex in the truck suspension, naturally, the cab tips a bit when you step up. You are 275, so you will see more flex. If flex is a key concern of yours, you might want to avoid any of the thinner look aluminum boards - your call.
The steel brackets use two factory mounting bolts per bracket. The brackets appear to be pretty beefy. The bracket is a type of universal bracket that has four holes. It is like a Isosceles trapezoid shape so even though it only uses two of the mounting bolts per mounting point, there is a large flat area against the inside of the truck rocker panel. All six brackets are identical. For the middle bracket, you have two options where you can mount it with maybe an inch of difference front to back, due to the location of the mounting bolts on the Ram. Like an inch closer to the front bracket or an inch closer to the rear bracket. If you are concerned about flex for you, you should probably install it on the bolts closer to the front bracket - shorten the distance between the front and middle mounting brackets - might help a bit with flex. A pic of the middle mounting bracket is below. Not a great pic, the ground is wet this morning and I am 'at remote work in my work clothes appropriate for zoom meetings'. LOL
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The black top appears painted or powder coated. It is a non slip finish with groves that run the direction of front to back of the truck. The non slip works. For example, they are wet this morning and they are still non slip if you try to run your shoe across them or even your finger across the wet surface. I was worried that it might be a rubber cap glued on, because we all know that glued stuff comes unglued over time and with extreme temps. So I was relieved when I saw the actual finish. They have the same kind of finish black painted finish on the bottom of the board which should help a bit with noise/singing from splash that you sometimes get with unpainted aluminum running boards.
I am very happy with the boards. Now, at 210 lbs I am not as strong as your 275 lbs. If you still want to go with any of the aluminum boards, you may want to verify if you can return them due to what you would consider to be too much flex. The install process is so fricken easy, It wouldn't be much of a pia to take them off and ship them back.
EDIT: I just ran an errand in the truck. I noticed that where I step to enter the truck, the ball of my foot is basically over the front bracket. The back of my foot is a few inches behind the front bracket. Basically, I am stepping with my weight on the running board on top of and just behind the front bracket. Basically the little bit of flex is bracket flex. You will get some slight board flex if you step in the areas of the board that are right in between any of the brackets. I consider it slight and normal.
Why am I adding this? If you get a stiffer non-aluminum board, but the brackets are similar to these, you will get the same bracket flex. So you may need to pay more attention to the brackets than the actual board. Like I said before, these brackets are pretty beefy to me.