Old Tyme Body Work

I would rent or borrow a mig to finish the job.
There's no way that I would use an oxy/acetylene torch to finish the job, there will be way too much warpage.
That is of course, you can't get to the back side of the panel and immediately and hammer and dolly the weld zone every 1/2'' or so which is also known as ''hammer welding''.
When you ''hammer weld'' you are actually relieving the stresses caused by warpage and making the weld more solid as well.......think of power forging and how it works to get a mental picture.
I would recommend against using a brass rod on thin mild steel, especially an external body panel.
How brazing works is almost like glue, it is joining two dissimilar metals together by when the torch heats the metal it expands and the brass flows into the pores and when it cools, it locks the two metals together. (kind of like velcro)
Unless it is done perfectly, it is quite weak and will still warp the sheet metal considerably.
And to top it all off, I have never seen a panel that was brazed on last long term because the use of dissimilar metals won't allow expansion and contraction to be the same at the ''weld area'' and any filler or paint will crack and peel off. And it's not a strong system either because of the tensile strength of mild steel is more than double than that of brass.
It was used a lot in the '60s and '70s before mig welding was popular in body shops.
It was fast and easy, and not many people were caring about the strength of the repair, as long as it held and lasted for a year or so. And then they would trowel on the bondo to fill in the warped panels that were as wavy as the Pacific ocean and call it done.

This is my 2 cents worth, and BTW, I am a journeyman welder that has seen a lot of cars over the years in the junkyards crashed, rusty, or whatever, and this is how I base my claims.