There are two ways (I know) of doing this. One: Cut your road drops out, re-slot the chain/seat stays, clean, and braze the track drops in. Two: Heat your road drops until the fall/can be pulled out, clean, and braze the track drops in.
Problem with One: If your rear tire has little clearance you will be limited by the amount you can cut off.
Problem with Two: Doing this can compromise the stays as the heat required to melt the brass can be bad for the steel.
Good luck, oh and just google track dropouts for suppliers Nova, Henry James, etc.