Jonald,
The supports are similar but not identical. the gmc has a bracket that bolts to the top that goes overtop of the radiator as mentioned above. It is removable. The Chevrolet has two cross braces that run in an "X" in front of the radiator. I'm not 100% sure of their function (perhaps because of the Chevrolets different grill mounting). I can take a look at my trucks as I have both gmc and Chevy clips sitting off their trucks and confirm. You can make the gmc work I'm thinking. For my GMC's mock up I used a Chevrolet support (had to remove the top bracket- but it is easily tack welded back on) because my GMC one was shiny and painted already. So I know it works in reverse.
WARNING!!!! CANDIAN GMC's used a Chevrolet engine. If it is in fact a GMC engine in there, then my information is NOT applicable to US GMC's. As Ed said the gmc mounts the rad further forward. Not sure if the US gmc radiator is different, or if the gmc CORE is different than the Chevrolet. WARNING!!!
The two "boomerangs" I'm suspecting are your front fender braces. They mount to the side of the core support (inside the front fender and outside of the inner fender) and bolt to the front fender on the inside of the outside lip. Look at the pictures in this tech tip:
https://www.stovebolt.com/techtips/bodywork/front%20fender%20support/index.html
