210mm Heliar Question

I recently picked up a21cm Heliar and its mounted in a copal #3, the rear cell actually stops the aperture blades from moving because the glass reaches too far into the back of the shutter. I obviously need to place a spacer of some kind so my question is how far can I go? I don't know what the spacing was when it was in a barrel mount if it was a barrel, it could have been mounted in a compound shutter I suppose. Mine is the post war coated variety. As the cells become farther from one and other does this change the focal length of the lens. Is a few mm one way or the other negligible?

Thanks in advance for any help.
SE

Put the lens backin the barrel and measure the length from back to front. then put it in the shutter and shim it until the distanceis exactly the same.

Or you could send it to SKG and have him mount it correctly and be done with it.

This 1964 Voigtlander catalog http://www.cameraeccentric.com/html/info/voigtlander_2.html lists the 21cm Heilar as being in barrel or Compound III.
The Compur Shutter repair manual http://www.suaudeau.eu/memo/rep/compur-repair/02-05.html lists the dimensions for Compound shutters on back side of page 2 of the Specifications.

The question you need to ask is does the lens focus as it is? Or, will it focus as the groups are separated?