Eric,
No, a propane torch will not melt brass braze material, not hot enough. If I read your post correctly, you need not preserve or be ginger with the quarter/jamb braze since you will be putting these both in from your new pieces, correct?
What I did was be harder on the jamb and preserved the 1/4 end/edge as I was not changing that out. The best way (short of melting it out with an oxy/acetylene setup) woudl be to use an air chisel with an appropriate bit, if you do not have one of those, a hand chisel and hammer. Work the middle of the weld and try to break the two from each other. If you have to preserve the area (why escpaes me as I mentioned), let caution be your guide. If chiseling doesn't work, try a dremel tool (and be prepared to spend a lot of time!). If something isn't working, figure out why and change what you are doing or try something else, when there is a will (and some skill), there's always a way!
The other thing I wanted to mention/ask, is what you used/did to do the cowling that made it take forever? A good spot weld cutter is all that it should take, granted drilling out all those welds is monotonous, but the results are gratifying! So said the guy who has drilled apart almost his entire car and put it back together!
I am here to help you if I can.
Happy Thanksgiving and Regards,
Bob