It doesn't matter about the bearings between all those GL models as you will still have to press in the 550 stem into the GL bottom tree.
The GL swap is not a direct fit for a 550. It is for a 750. The stems are different lengths.
For the forks, just buy a set to tapered head bearings for a 550 and press in the 550 stem to the GL bottom tree. You will also need the entire break system from the GL or fit something a bit more modern on there. I'm not sure about the wheels on the 78/79 GL. 78/79 were comstar's I believe but I could be wrong.