Bill,
My 343 has a 25mm shaft with a 30hp motor. Pity that you couldn't get the size information and source from Beneteau. I would call Ward Richardson or Debbie at Beneteau USA and pester them until they get the information for you. They have to have that info on file in their system. Another source might be to talk to Michael James at Murray Yacht sales to see if they can get the info from Beneteau; they can hopefully get the full attention from Beneteau as they are a big Beneteau dealer. Another source would be Annapolis Yacht Sales (Beneteau dealer); they have a very knowledgeable parts department and good inventory of parts. I have purchased from them in the past and they were very helpful. If all else fails, you might check with the mechanics at Sintes boat yard at West End or Aaron at Seabrook boat yard to see if there is a local prop shop that could help you. The important thing is to get the correct internal diameter to fit the shaft, the correct diameter of the blades , number of blades, and pitch that matches your motor and hull displacement otherwise, the motor isn't going to run at the correct RPM's at hull speed.
Hate to see you have to have a new one cast; guess that will cost a bundle. Good luck. David (NOPS)