So these are now staying put. I've ended up ordering a Stronglight 200lx 52/38 with french pedal threading for the new frame. The Look! pedals can go on that, and these cranks can stay where they are. Before anyone worries about any cloak and dagger stuff, I'm not planning on selling the frame - and if I ever do it will be with full disclosure that the stem is stuck and the left hand crank's threading is stripped.