The Moto Guzzi V35/V50 bearing cups from Ed Milich were definitely worth the C-note. They will fit the Benelli 2C frame perfectly. (I haven’t pressed them all the way on yet, but rest assured they will seat fully.)
Unfortunately my earlier statement, “…this could go together without any machine work or custom fabrication. That rarely happens in reality, however…” is proving accurate. The GPz750 steering stem is too large at the bottom bearing to insert fully into the new bearing cup. The witness mark annotated in the second photo shows where it bottoms out at a diameter of about 26.5mm. Looking at my diagram up a couple of posts, the conflict should have been obvious to me. If I end up doing machine work on the lathe, it will have to be a custom stem, not widening the cup; the inside wall of the cup would end up perilously thin.
Fortunately, I haven’t spent a dime on a front end for this yet, so switching to a different set of steering clamps is no big deal.
Ideally, I’d like to find a front end that use 30205 bearings top and bottom. Fortunately, this a fairly common combination for European bikes, including mid-size Aprilias and BMWs, as well as nearly all older Moto Guzzis.
No Japanese bikes use 25mm ID bearings as a pair, but a lot of Honda dirt bike stems are 26mm diameter top and bottom. 26x52x15 tapered roller bearings are available (SKF 639174, wheel bearings for old Fiat 500s and 600s). Thus, these are all possible options: