I talked to a guy that was doing this years ago, and saw the car. He swapped to a GN nose and doors, using the Camino's glass. the Regal's have a body line that the Camino's don't. He solved this by having a friend who is a body man add this line to the rear quarter, just an inch or so, and had it curl up to the rear side window. Looked good, and is probably much easier than blending two fenders together. That is the way that I would do it.
By the way, the El Caminos are the same width, but they are longer. If you want to change one over, you will have to use the rear and driveshaft from the Camino, or if you put the 8.5 in, then you will have to have a custom driveshaft.