The way I refabricated the mounting bracket, the antenna leans slightly backward when extended, as opposed to the OEM antenna which used to extend straight up. An electrical engineer could argue that this slight angle could compromise radio reception very slightly, but I've never experienced any reception differences from between stock antennas and the Radio Shack replacements. Below are the Radio Shack antenna, on the left, and the OEM Mazda antenna with the mounting bracket installed, on the right. The OEM bracket has four removable bolts on it. The top of the OEM bracket can be unbolted with the top two bolts and re-installed on the RS antenna.
All images below are clickable, by the way.
The mounting bracket has a 180 degree bend on the bottom. I partially straightened it to a 90 degree bend, and snipped a portion of one of the bolt holes to accommodate the Radio Shack's bottom mounted bolts. The finished product. You can see that I had to bend the bracket to lean it over in such a way that the mast holes and bracket bolt holes would all line up to the chassis. If I had, the bracket would have mated to the Radio Shack unit with little more than unbending it 90 degrees and snipping that little notch out of the bolt hole.