Your mechanic is right, the 20Vs came with equal-length driveshafts, which means that the driver's side has an extension-shaft which rests in a carrier bearing bolted to the block, whereas ADM ones are unequal-length so the driver's side is heaps longer and needs a rubber damper on it to reduce vibration.
If you're struggling to find equal-length driveshafts then there shouldn't be a problem running the unequal ones to get you out of a bind as long as you remove the carrier bearing, you'll just suffer from a little bit more torque steer.
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