You may have found out by now that the mounting ring has (or
should have) a shoulder that fits into the lens-board hole
and that everything is just the size it needs to be.

If, however, the hole is too large even with the mounting ring
then you _may_ have a small problem.  If you can tighten down
the mounting ring and there are no light leaks all is OK.  
If the mounting ring sort of tips/slips into the hole or light
leaks then you may need to make a 'washer' out of AOS
(Any Old S---[Stuff]).

The oversize hole/shoulder on the mounting ring is good
engineering practice.  A similar shoulder is present in
the upper unthreaded portion of a bolt.

The reasons are:  

1) The shoulder can fit into the hole with a tight
  clearance and hence precisely locate the lens
  (a pretty useless feature on a view camera where
  the whole purpose of the thing is to move the
  lens off-center);

2)  The shoulder prevents the threads on the lens
   from rubbing against the sides of the hole and
   getting damaged (if the lens board is metal).