Realizing that my example isn't a 64.5 Mustang headlight housing but the principles are the same...this Muggy Weld REALLY WORKS!!!
Much like soldering, you need a low heat and just watch the videos online to get a little reassurance before you tackle it. I would suggest starting on a project that does not require perfection, for me it was a crack on a rear fender extension. This gave me a little practice to go ahead and try the bigger repair...that repair is the one pictured here. (and a few more shots posted earlier in this thread) You can see the broken out, missing area below the headlight housing where the valance attaches (with a J-clip). I simply filled the whole area using wood as a backer, then I flipped everything over and did the same thing on the other side. Some of the pictures I previously posted in this thread, demonstrate a little bit of what I am talking about.
"Black (burned) Flux" is NOT what you "want to see" as a few of my shots show...Root Beer Brown is best
I would say this stuff, structurally speaking, is stronger than the pot metal itself. When sanding it into shape, you will notice it is a bit "harder" than the pot metal. This essentially translates as (IMHO) a much stronger repair than any other product anybody has mentioned so far for doing such repairs. I would say for the 64 .5 cars, all of the issues that are notorious with them, would easily be tackled by using this product.
All you need is a little practice. If you find yourself struggling with getting the job you want, go back and watch more of the MuggyWeld videos, There are many to watch. I am pretty sure that one of them will show you what you might be doing wrong if you are having such troubles.
For the pot metal repairs you only have to buy the "Alloy 1" product. I bought the "MuggyWeld Combo 4" but have not tried the "Alloy 5" yet.