I part answered my own question whilst dozing in my chair after Christmas lunch. These J8s seem to come with 39-42 adapter, so they can fit in an m42 body. And of course, in a reflex camera as long as the image is in focus on the screen, it should focus on the film, or else how could extension tubes work? I hadn't thought it through properly.
So I'm wondering now if anyone with m42 has successfully used a J8? But I'm now also looking at Helios, although these are not Sonnar. Thanks for your input.