There are a couple of references to some mods to 3/60 cpu
boards, originally from, I think, Peter Koch, of the sun3archive
fame, where he mounted an internal drive on a 3/60 cpu board
in the place where the drive holes are, and ran a ribbon cable
over to a dip header just behind the usual  db50 connector.
That is not apparently the p4a or p4b connectors.  I just pulled
my 3/60 cpu and the dip pads are there, but it is unlabelled.
My suspicion is that there may have been some sort of 3/60
version that was never marketed, but which had a drive in
the internal space provided.  I just flipped the cpu over and
that 50 pin dip pad is labelled J6.  There is a pix of the mod
on his machine, somewhere on the internet.  I think it is at
www.sunstuff.org/hardware/systems/sun3/sun3/3-60/harddrive.shtml.
The difficult part seems to be adding the 50 pin dip header to
the pads.  There are also some pads to draw power for the drive,
although I can't find them offhand on my board, or see if from his
pix.  But, the potential is there for adding a decent output connector
or for adding an internal drive to the 3/60.

Bob Keys