I think from at least the early 80's on there is pretty broad compatibility between downtube shifter bosses, and therefore between clamp-on adapters. Shimano made some bosses early on that aren't compatible with anything else and I'm suspicious of those Huret clamp-on adapters because the studs are round. I think you need something with flat edges. Somebody used to sell Huret adapters that were specifically modified for Shimano compatibility, but they may all be gone now.
I know for sure that clamp from the Shimano indexed levers that thinktubes linked to will work for you. I've bought a set of those shifters for a very similar purpose. For whatever reason, it's usually cheaper on eBay to buy a clamp with shifters attached than just the clamp.