Now I remember how it worked on my Peugeot with the DT cable stops...
In the pic of the Simplex clamped on retrofriction shifters above, you see large chromed washer/metal disc just inboard of the shifter lever drums between the lever and the clamp. These discs are proud of the lever drum surface and it was the part that had direct contact with the cable stops when the shifters are clamped on. These do not turn with the levers as they act as stops (through tangs that stick out towards the front) for the lever swing when you push the lever fully forward so the levers are free to operate even when these parts are contacting the cable stops. There is a very good chance that the same will work with you frame if you use the Simplex retrofrictions or another brand of shift levers that have a similar thing going on.....as Simplex retrofrictions are now quite expensive these days.
Chombi