That type of peg is designed to hang in a clevis, so a mounting bolt isn't needed. What is needed is the toe lever connection to heel peg.
Since you've bought those "pegs" you're driving yourself in a certain direction if you want to use their mounting base, and in my opinion creating a tad tricker design and fab effort.
Your sketch can be simplified. Eliminate the whole threaded rod assembly by a using a standard SS bolt coming thru the back of the plate and into the peg. Between the peg and the plate from the outside, you need the bronze bushing to act as a collar inside the peg, and perhaps a small spacer to move the peg outboard. Also, spinning on that same shaft, you'd create the "ear" that the toe peg (shift side) links to. Unless you plan to create a 2 piece rearset that uses stock type shifter arms and brake arms?
Perhaps you could sketch something up completely and that would help. Also, SS or aluminum are both strong enough. You just need a thicker piece if aluminum. Tube OD is 1" so you've got plenty of surface area to attach to. If you're planning on screwing hate plate to the mouthing tabs on backside of tubes, I'd use 1/2" aluminum billet blocks, milled or cut down. Flat sheet is fine too, but I'd want perhaps 5/8" in aluminum. For SS, 3/16" minimum.
A benefit of using slightly thicker material helps position the rearset "outboard" of the tubes for your ankle to clear the bike. It also helps align the shift side, even still you may need an offset shift arm.
Then you need to work out actuating the brakes and clearing for the kickstarter.
