Jarno

Very nice bass!
Tuners will need to be this close together to allow the strings to be pulled in as much a straight line as possible. You can make the headstock as big as you like, but if you have tuners on each end, they will need to follow the taper of the neck, and it'll mean they are close together. They would be further apart if the nut would be wider, but that's not to everyones taste.
For an appaling (IMHO) example of tuners too far apart and strings being pulled in weird angles: http://alembic.com/club/messages/411/54184.html?1323560356
(I'll put on my asbestos suit now, after all it IS an Alembic )

Which Schallers are they? The 16mm of the Ferrules, is that the inside diameter? Do these fit the 11/16ths (17.5mm) holes in a standard Fenderish neck?