Jim, it's all 16-gauge cut from a standard-sized 4x8 sheet. This stuff has a chromed side and an ordinary brushed-finish side.
Here's a sort-of how-to, if you're interested:
Required tools are generally a scroll saw, a whizzer with a ScotchBrite pad on it, tin snips, cardboard for templates, a Sharpie, a one-meter straight-edge, a square, and a spray-paint can lid for curved corners to follow with the bead-roller. The break and the bead-rolling press are readily available from Harbor Freight for not a lot of money.
He generally uses 1/8" rivets, a pneumatic drill with a corresponding bit and drills the holes off the chassis.
That process is part of the finish work, but has to be done before rolling and folding; to make the holes look evenly measured, he starts at the corners, .5" in from the edges, and then cuts the distance from one corner to the next in half, drills another hole and then keeps dividing the distances so the holes are evenly spaced and all half-an inch from the edge of the sheet. He chases the holes manually with a larger drill bit to de-burr them.
We bead-roll the ridges in before any bending of the shape we need, and then use a manual metal break to make the straight-line bends.
The polished side has a plastic film on it that he makes his Sharpie marks on, and when he's done folding a piece, he hits a rag with Brak-Kleen and wipes off the marks. It's all him from then on, but I'm learning as we go. (I fiddled with my own tins for a while, and then he tweeked them a day or so later to make them perfect. It's kind of like sheet-metal oragami.)
After the fitting adjustments with the snips and pneumatic cookie, he clips it into place with kleekoes (temporary rivets) and works on whichever pieces touch the first one. After he's done with all the little pieces he needs, he takes them out and peels back the plastic before he rivets them in for keeps.
He generally puts larger pieces over smaller ones, like in the shot labelled 427 MCYL (above); it's a small finish-piece tucked into the corner under the fenderwell piece, making up the gap between the firewall and the fenderwell. You can see the rivets, but not the demarcation between the sheets when he does it that way.
These are old, but they're from last year's efforts on my car: