If you don't mind, I'll give you my thoughts.
I agree with the things Brad mentioned about adhesion problems. And he's right, brazing a panel was considered a sloppy but passable way of getting one "out the door" decades ago. It was never the right way, so I eventually learned to gas weld with steel.
But back to your repair! Because I restore lots of these old cars, they often have a shady past of repairs hidden by filler and paint. Here is what you might possibly do, it has worked for me.
I am assuming that the back section was replaced and possibly (hopefully) overlaps the original panel. If so, you need access to the back of the panel to determine how much over lap there may be. I've seen as much a couple of inches on some cars. Sometimes there are burn traces on the backside, but they were often hidden with undercoating.
If there is enough over lap you can possibly make a cut through both panels, just behind the leading edge of the top panel. This does two things. It gives you a way to remove the repair panel while you dress the edge of the original panel (remove brass and remains of repair panel with grinder or heat), plus it should leave you with two clean matching edges to allow the removed panel to align with a clean original panel. Now the two can be butt welded with no brass to interfere.
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