Solder isn't really made for filling gaps, if it is hot enough to stick it is hot enough flow. That said, you may be able to fit some brass in your slot, file it exactly to fill the gap, and use some silver solder that is made for working on gold-filled (which happens to be brass colored) I got mine from a jewelers supply catalog. I think it flows at around 1100-1200f or so, but don't hold me to that number. If you've got experience silver soldering and make the appropriate filler pieces it could work for you.
-Page