Drop out retros are not difficult, but do take time and a high degree of accuracy to complete correctly.
The dropouts you have are not difficult to remove...after stripping off the paint, place the frame in a stand with the headtube pointed toward the ceiling. Attach a heavy set of vice grips to the dropout and begin heating with a oxy/acet torch evenly around the piece...the dropout will fall off once the brass is liquid.
The tube stubs will then need to be cut and slotted to fit the new drops, which need to be held in position parallel and in phase. A fixture makes this much easier, but plenty of time, patience, and ingenuity can suffice.
Insure that the slots form a tight fit and then clean, flux, and braze.
I like to use these drops from Paragon Machine works, as they have the horizontal slot but still allow for a derailleur if you desire.
cheers,
rody