To my knowledge the Parallel has never changed. I have some very old examples, probably greater than 15 years, and two brand new ones. The parts all look identical in dimensions. The M2, then, may have changed at some point but I don't have any data on that.
I hope part of this plan involves feeding the newly constructed Franken-pen with Pilot cartridges, because I would not trust an eyedropper pen with the wrong feed shoved into it not to leak...
What's wrong with the Parallel body as-is? Too long for you?