As far as I know, the neck pickup should be exactly under the (imaginary) 24th fret and the bridge pickup as closer as possible to the bridge, that gives you the wider range of frequencies... and, since the bridge location is determined by the scale length, the only question i can see here is the location of the scaling center, which is the 20th fret (where the body starts). If you have access to a vector drawing program, like Illustrator or Autocad, you could roughly draw the guitar on its original size and then overlap the baritone fretboard as a guide, centering both drawings around the 20th fret. Then start scaling the original drawing, using the 20th fret as center, until the original fretboard is the same length as the baritone one. That will scale the body in a proportional way as well... but maybe you'll have to reduce a bit afterwards due to a more narrow fretboard. Not sure if I've explained well, you can do it at eyeball too...