On a second though, adding a doughter card must be going to rise the
cost and time to design significally, isn't it?
No. The signals on the SC and PC chips (with the exception of interrupt
levels > the first one) are 100% compatable, so you can go with the
standard 179 pin PGA on the main board.
4000PC, 4200PC, and 4400PC chips are plug compatable, change them, the
crystal, and setup for the serially loaded boot data which determines
the clock divisor and access pattern for the memory interface.
For the MC/SC series chips or a surface mount 4200, you can go
with a daughter card that routes the pins to the right place, just
like AMD does with their 386 chips, HP did with the 9000/400's which
could take either a 68040 or 68030 + 68882, etc.