See if an SD front shaft is sufficiently shorter than the 420 front shaft. An entire SD prop shaft won't work because the 420 is more than 1/2" longer in wheelbase. If an SD front shaft doesn't work, maybe a 300SE or 350SD front shaft is the right length.

If all you need is a half inch, maybe you can shave a quarter inch from the face of each flange/yoke.

Sixto
87 300D