An ambitious project! What great fortune you've had in finding MS parts for it.

Count me among those who are cringing at the thought of an eighty year old original Mannlicher Schoenauer stutzen stock being cut, but it's your project. A sixty centimeter barreled half stock is what you desire, and that's what you'll create.

If only there was a time machine available. With that (and $175.00 or so in 1939 U.S. dollars), you could have ordered one 'fresh'. My M1910 takedown has a barrel of approximately 60cm.

Note the various barrel lengths available and 'custom' options.

This is from a 1939 Stoeger catalog (likely compiled and/or printed in 1938).
I believe the '40 Stoeger has the same page.