As soon as I read M42 I thought of SLR's and then realised that there are no Jupiter 8's available for them.
The problem is that there is not much clearance behind a J-8 for the mirror box. From the film plane to the glass at the back of the lens is 27.75mm (at the infinity focus); or the aperture blades are 42.14mm from the film plane to give a better idea.
So the obvious answer, for me, is buy a cheap USSR made film rangefinder or a dear Leica digital, or something in between. And, of course, there's the Contax/Kiev route to explore but I can't think of anything else; although there may be something in the CZ range. Anyway, I'll wish you luck.