My 2 cents for what it's worth is:
If you are going to the bother of doing the work, i would suggest that you cut out the rotted metal and either weld in new metal, or at least use a panel adhesive and do it right the first time.
It looks like it was ''bondoed'' up before, and then the paint was spotted in.
That's a very nice looking car you have there, it would be too bad if you went to the trouble of a patch job and the rust growing any more than it is now.
And it will come back to haunt you, and the rust will even be worse than it is now, you can mark my words on this...........