Haighter, can you post a picture of your derailleur?
Often, the derailleur will bolt on to the clamp in the same way it will bolt on to the bracket. If that's the case, then you'll just need to purchase a bracket that will fit on your frame correctly, unbolt the derailleur from the clamp and bolt it to the bracket.
Walt
Here is a picture of the old one.
And the new one:
So, it's my understanding that if I get a dereailleur billed as "braze-on" that I can take the mounting bracket off of this old one and use it with the new one? FWIW, I was thinking 105 FD.