I believe you need to swap your upper and lower mounts/pads to the big block mounts. The should fit over the tank and put your shroud mounts where they are supposed to be. I just installed a Cold Case rad and had to do the same. Fits like a glove. The factory mounts go over that tank/core seam. Or you could use something like this link from Summit. They used this style on a Week to Wicked build a couple of years ago (red and black C10).
https://www.summitracing.com/parts/s...BoCqB8QAvD_BwE