Originally Posted by Chombi

If you are lucky, the stops would not interfere with a clamped on DT shifter, especially if they are high enough on the DT that they are mostly behind the shifter drum and out of the way of the cables.
My 1982 Peugeot PH10S had cable stops for stem shifters on it and I bought a set of clamped on Simplex retrofridtion shifters and mounted them just in front of the stops, using the stops to keep the clamp from sliding back. In my case, the shifter drum and cables did not make any contact with the stops. Only thing that contacted the stops was the back edge of the shifter's DT clamp so everything worked out well without me messing around with the cable stops at all.
I'd give that approach a try first before deciding to cut off anything from the frame.

Chombi