Of course, you could always use Evergreen's scribed car siding, which was made expressly for this purpose and scales out to exactly 3 1/4" spacing.
Another point... Rex was in error to say that you should scan the plans and scale them up and measure the length... this would magnify any errors in either the plans or the scanning (often scanning will "stretch" one axis more than the other). This is why you should never scale anything from a plan, and why good plans are always fully dimensioned.
Bill Daniels, P.E.
San Francisco, CA