I need to know John if the gt550 block would fit the cb500 and vice versa, the only way to do that is to either put them on top of each other and see if they match or test fit them and see. However I know how much work that can entail and it’s why I was a little reluctant to ask. If the 500 was coming apart for restoration then not so bad, next time you’ve got the rear wheel out of the gt it’s a simple job of comparing them.
The info may come in very handy for members restoring the cb500 which is why I ask, it can wait though until it’s possible for you to do it.
Must admit they look the same but there could be a very small size difference that doesn’t show until you try to use one in the other bike.