I opted to find the "chassis specific" crossmember so that other GC parts would hypothetically fit with no questions. (H-brace and aluminum control arms)
The GT Spec brace is listed as a GD part. The LCA mount in a GD crossmember is a bit wider than a GC unit, so I had to make some spacers to take up the gap between the eye of the brace arm and the forward ear of the LCA mount. In a situation where a car with GC lower control arms are installed in a GD crossmember, these spacers would be installed between the ears adjacent to the bushing.
That said, using a GD crossmember in a GC car may change where the rear mounting points of the H-brace end up due to the wider LCA bushing mount. Based only on my experience mounting the other brace (GT Spec), my guess is that it is the forward ear of the LCA mount that is pushed towards the front of the car. So if the backside of the rear ear of the LCA mount is in even remotely the same place, the rear mounting points of the H-brace should end up close enough that the bolts would go in. And as AcquaCow's picture shows, with a GD crossmember, it bolts up.