I was hoping the OP would have chimed in as to which roof it was, as it narrows down what to share so as not to be confusing.
First either method needs preparation so no contaminates will cause cracks or lifting.
Vinyl roof, body filler is just fine, as in the event there is cracking of the patch down the road of time, it will be covered by the roof. Factory method.
Painted Roof.
Many ways to do it. Lead is preferred by many, as it stretches with the body flex and that is why the factory used it. Yes over time it still may crack, but most have survived 40+ years with no cracks. Some don't like lead due to the toxicity of it and choose other methods that have worked.
I have seen where a strip of metal is welded in over the seam allowing a thinner coat of filler, versus filling up the deep seam of about 1/4-3/8 inch. Obviously thinner filler equates to less likely-hood of cracking.
I had not considered the brazing, but in thinking about it, it is softer than steel , and easier to work with. It will stretch and flex as well. Most likely a good choice other than lead.
I have heard of guys using JB Weld. I am not sure about this as it may be quite brittle especially after being subjected to sun light.
One thing I would never do, is cut the metal and but weld, if this is done, all the strength of the overlap is lost , thus creating an even weaker seam.
Hope this helps, good luck on the method you choose.
