I've done these before. Use the Kadee #5 (buy a pack of #148, much cheaper that way) and the draft gear box #242.
If yours has the couplers mounted to the truck, simply cut them off.
Kadee suggests glueing a thin piece of styrene under where the gear box will go to shim it up. Wait until that's dry and then glue the gear box body to the frame. If you have some small enough wood screws, (2/56 IIRC) when you install the couplers and the cover piece, add them for a better grip on the body. I've seen glue-on couplers pop off with little effort.
On some rare occasions, you may need to use a coupler with a different shank to properly adjust the height.