I think you are missing my concern. A washer at the fixed cup is not a problem. As it shifts the spindle over the same amount, the amount of engagement of the lock-ring over the adjustable cup may be inadequate. There, I want to see full engagement especially with an aluminum cup. How this all works out is really the sum of the variables, the bottom bracket shell being the initial concern, they often get faced, some more than others, more metal removal on the drive side the bigger the potential problem.
Campagnolo in the Record bottom bracket era made spindles with dimension modifications and stamped as such. Bypassed by the Corsa Record era. C Record and Chorus are close but not fully interchangeable.