If they are standard 40mm x 40mm fans (which I am almost 100% sure they are) the center to center distance (not diagnally) is around 32.5mm.
EDIT: Andy's original shifting tranny had these mounted on it. That PDF should have the hole spacing too!
EDIT 2: Looks like he has it at 1.265", which confirms the 32.5mm within some uncertainty. A 4-40 should have plenty of wiggle room

__________________
-Travis Covington
2008-2017 - Engineering Mentor of
Team 254
2001-2008 - Engineering Mentor of
Team 968
1998-2001 - Mechanical Director/Driver/Member of
Team 115