www.tanfoglio.it for gun descriptions.
Stock II and IIIs now come with the exact same sear setup - IE same trigger.
Tanfoglio large frame guns are designed for larger, longer calibers such as 38 super, 10mm and 45ACP, but they do make them for smaller calibers - 9 and 40.
Small frame guns are only available for smaller calibers- 9 and 40. Larger frame guns often require non-spec (longer) ammo in 9mm for perfect feeding reliability.
In Canada, for 9mm, we only import small frame guns - unless it's special ordered. For 40, by default, we bring in small frame for the Stock Models and Large frame for the Limited Customs (that's Tanfoglio's default)
(In the US the importer there brings in only large frame for all calibers as they don't want to deal with different frame types)
We stock Stock IIIs in 40 in small frame (so it uses factory spec ammo easily) and have them on the shelf now.
As for 40 - well, it's pretty rare that you see someone shooting 40 in production. You should ask yourself why you want to re-invent the wheel, while so many before you have chosen otherwise. I'll sell you whatever you like, though.
Must to do - get the Tanfoglio Xtreme parts and lighten the trigger pull (Firing Pin, Hammer, FP and Hammer Springs)
While I shoot Stock IIs; I don't really see much advantage out of them compared to a Stock III. My usual recommendation is a Stock III.