For mine, I just loaded SP3 and V3 and applied the same pose to both.  Then hide the SP3 head and V3 body.  Tweak the positioning a little to get both to match up as best as possible, and that's it.  Fairly quick and easy if you don't have the time (or skills) to actually merge the meshes, create new necks, etc.

"You don't know what we can see
Why don't you tell your dreams to me
Fantasy will set you free." - Steppenwolf