When the tubes aren't mitred, I'd rather have them protruding into the shell than not. When they are protruding into the shell, you have a stronger joint due to a higher percentage of engagement with the tube collars on the shell. This is why good frames have mitred tubes. There's 100% engagement and maximum strength, provided you get full penetration of the brazing material.

If you have problems finding someone to cut French threads another option is to have the shell reamed and threaded to Italian standard, which is far more common.