The image attached to this post is the upper corner on drivers side. Brass is used, probably many years ago. Maybe brass could be used now also, using a 300 watt soldering iron and flux



 

Yep, looks like brazing to me, which is oxy/acetylene welding with a brass rod, so it is indeed brass. Very old school. Another old school technique was 'leading', which is using wooden paddles to shape melted lead, similar to the way bondo is used today.
I have a few spots like that on my '67, which I plan on patching with sheet metal using a MIG welder. My little Hobart welder is invaluable, even a cheaper wire-feed welder would be a big asset for anyone who does body work on a semi-regular basis. Using fiberglass will indeed work, but if I were you I would find someone with welding experience or borrow a welder and learn 'on the job'. Spot-welding small patch panels isn't that difficult even for a beginner. Having a fiberglass repair would bug then heck out of me even if it were hidden... but that's just me & my OCD talkin'.

Glad to hear you are going with welding. Do it right the first time to avoid issues later. Problem with rust is that it always seems to creep back. So although fiberglass is a good temporary fix, the rust most likely will return in those problem areas where water and humidity collect and you end up with the same problem later.
Be very careful with welding. Sometimes, there is hidden rusted areas on the other side of the metal you can't see, that will burn or open up during welding. So what begins with a small hole, could end up "burning open" during welding. Further, the rusted areas you have cleaned up are thinner now, and are susceptible to burning through.  I have learned that what appears to be a quick repair sometimes develops into a more extensive repair job. You may end up "patching in" some metal to repair those holes. 

I agree that an oxy-acetylene torch is not the best option for what you're doing. And those little tanks on the mini-torches don't last long at all. I have a full size oxy/acetylene outfit and I use it mostly for cutting and heating up stubborn nuts & bolts. I do the majority of my welding with my Hobart 140 MIG/flux core unit.

For a beginner I would recommend doing what Daze did and start out with a simple & inexpensive wire-feed flux core welder like this:

http://www.northerntool.com/shop/tools/product_200632003_200632003

That one doesn't use argon gas, which means you will have some popping and spatter, but the welds can be cleaned up with a grinder and it works fine for patch panels where you are mostly spot welding. My Hobart has argon but I don't always use it in order to save on gas. Later on if you find yourself welding on a regular basis, you can upgrade to an argon MIG welder as Daze did.

The best way to fix areas like in your pics is to cut out the entire sections where the metal is thin and there are numerous holes. Then cut & shape sheet metal patch panels to weld in. As mentioned, simply trying to fill the holes in usually results in bigger holes.