The duct is different in the middle. It is clearanced for the speedometer and two light bulbs. Here are some photos of the duct for the SS ( round gauge ) dash. Send a PM to oman. He modified his duct with a heat gun for some aftermarket gauges and that might work for you.
Yup I did do the modification mentioned.
VERY VERY EASY. Just don't get carried away with the heat. Main trick when relieving the duct. Get something shaped the way you want the area in question to look. I used moderate size peen hammer. Heat the area in question, press the ball peen hammer into the soft material and HOLD IT THERE. Make sure the duct cannot squirm around on the bench. After heating and pressing the "die" in place use your other hand to spray some cold / cool water on the area you heated. I used a Windex bottle filled with tap water. The cold / cool water "sets" the plastic in its new position, otherwise it wants to crawl 1/2 way back to the original shape as it cools.
Also be aware the material is THIN. Too much heat or trying to make too deep a dent and you will break thru the material. You just cannot stretch the stuff forever. I don't know how much massage is needed for the "conversion" of a non SS duct to work in an SS dash. What I needed was perhaps 1/4 or 3/8 inch more depth in a place where the SS duct had the reliefs shown in your pictures. That 1/4 worked fine.
If the OP needs a BIG relief I suggest starting by marking then heating a BIG area and "working" / "blending" / "massaging" the dent into the duct to get its new shape working from the outside of the relief toward the center. Remember you can't stretch the material forever and you cannot put a right angle in the material where the relief has to start. If possible get a "test piece" using an old duct. You can learn the tricks of working with the stuff without risking / trashing the duct you are going to use.