I'm going to use a Shimano 600 group on this bike, and the front derailleur has a built-in cable stop, so I have a clamp-on cable stop/guide for the down tube, which means the rear derailleur cable has to run on top of the chainstay. I have another clamp-on cable stop to replace the braze-on that I want to remove.
Is the oxy-propane readily available at hardware/building supply stores?
Just cut it off and file it smooth very carefully to avoid cutting into the stay. No reason to introduce heat where its not needed. Those Bernzomatic torches are more trouble than they're worth. I went that route and burned through a lot of $10 oxy bottles just building one frame. It really sucks when the oxy runs out in the middle of a joint and the straight mapp soots up the joint, requiring you to clean it and start over.
Keep the flame on the stop itself, avoiding the stay as much as possible. When the braze under the stop melts, knock the stop off the stay. While the braze is still liquid, use a wire brush to brush the bulk of it off the stay and you'll have minimal clean up afterward.
N.B. Air/propane is unlikely to be hot enough to melt the braze. If you don't have a fuel/oxygen setup, use the saw and file method to remove the stop.
I've been waffling over what to do with this frame ever since I got it, and I thought I had arrived at a final solution with removing this stop and adding the clamp-on stops to accommodate the Shimano 600 group.
Then I got a price from a local frame builder to move the rear stop to the top of the chainstay and add another one for the FD to the down tube - $50 - so that's what I'm going to do. Much cleaner look.