I'm actually doing something somewhat similar - I now have a second Custom House Little Junior body, which is between Yo and MSD size. I bought him to put with an MSD head, but it happened to be a rather
large MSD head (Leeke Oatmeal) and he just looked ridiculously bobbleheaded. So the Leeke boy is going to be a floating head for a while until I get an MSD body for him somewhere down the line - in the meantime he'll be victim to my faceup practice.
But I bought that CH LJ body to be a certain character who's around the same height as my other CH LJ boy - so an MSD body won't work for him! What to doooo...I looked for some CH LJ heads, but couldn't find many on the marketplace - none that would work for the character, anyway. Then I found a CH Ange Ai head, which is their yo-sd size. I think they're 27 cm tall and the LJ body is 35 cm. The head size is very close - LJs have 7.5 heads and Ange Ais have 7s. The neck is a teeny bit bigger on the LJ body, but I have a dremel and I know how to use it.
(I'll be enlarging the neck hole if I need to, not minimizing the neck - either could technically work but I'm afraid I'd end up with an oddly shaped neck if I tried it that way.)
But as Zircon said the issue could be pairing a childlike face with a more mature body - the LJ kids are pretty childlike too so I think it should work for my case. But if you want a childish MSD, there are plenty of those too, so it might be okay! If neck length turns out to be a problem, some sanding/dremeling could work wonders for that as well. Volks MSDs are only 42 cm which is shorter than most, but they're also a bit chubby, so I'd worry a smaller head would look tiny on them.
Sorry for long, rambling reply.