Thanks for the input! I wasn't going to take the elements out or anything so I don't need any special tools since it just twists off. I only thought to swap the rear groups but swapping both front and back or taking and viewing is worth a shot as well. I'll try all the variations and shoot a test roll and if all else fails, I'll get the glass cleaned.
I would set up a test -maybe cover a wall with newspaper (because of the fine detail in the print) clamp the camera ona good solid tripod and shot some test shots with a remote cable on a slow fine grained film with a wide aperture (a shallow depth of field shows up focussing issues much better) Then swap the lens elements and repeat.
The trouble with putting a film through in 'normal' usage is that stopping down, camera shake and the fact that each image is different makes it difficult to spot subtle errors. Might be a long time before you hit the conditions that show up a focussing error.
There is one exception to the potentially interchangeable rule that I know about - the 80mm 'S' has a different viewing lens construction from the taking lens. The 80mm f3.7 uses a different (Copal) shutter and may also be non-interchangeable. This is not to say that the viewing lens is always as good for photographic purposes, but anecdotal evidence suggests that both optics are of similar quality.