Pipe threads are tapered and 3/8" is 0.675" OD X 18 TPI
Here is a link to a chart for pipe thread dimensions:
http://en.wikipedia.org/wiki/National_pipe_thread
Electrical conduit threads are the same OD as pipe but straight instead of being tapered.
Here is a link to the chart:
http://www.home4c.com/conduit.htm
Since the plug seals on the shoulder, a Heli-coil or Keen Cert would be a good way to repair a stripped thread in the oil pan.
I have worked with a lot of Mercedes and BMW as well as VWs over the years and have had very few stripped drain plugs on aluminum pans. On the stamped steel I used to keep thread repair supplies in stock.
If can not come up with a thread repair insert and you want to make the ultimate thread repair insert for the 5/8" X 18 TPI, obtain a 1/2 " X 1/4" steel reducer bushing. Put a 1/2" coupling into a Lathe and screw the 1/2" X 1/4" bushing into it and face the end. Next drill out the 1/4" female pipe thread to 37/64" and tap it 5/8" X 18 TPI. Now drill the stripped pan to 23/32" and tap it with a 1/2" national pipe tap. Apply sealing type thread locker (Loctite 290) to the insert threads and screw it in and tighten it down. Screw in your 5/8" X 18 TPI drain plug with a copper washer and it will seal.
I used this trick to fix a lot of stripped drain plugs including steal pans, some times I would braze in the modified pipe reducer bushing on steel pans.
Here is a chart for tap drill sizes:
http://www.newmantools.com/tapdrill.htm
As far as the threads in an Italian built engine; they make attachments like oil pans and manifolds to the buyers specifications, that is why it is SAE. Chrysler wanted to fake us out, but it did not work, we figured it out.
Steve