I have a 12 gauge Beretta 303 which also has 26 inch barrel.
Excellent choice!! 26" isn't too short. I'd put the 4-notch (Imp Cyl) or 5 notch (SKT/CYL) choke in it. I've got a fair smattering of 302's, 303's, and 391's in 12 and 20 gauge. Bbl lengths from 24"-30". With the exception of turkeys, I don't ever go tighter than IC for any hunting or targets.
My 1st Beretta semi-auto was an early 303 12Ga w/26" bbl. I got it in the early 90's sometime. It's got an IC Briley extended choke I bought in the mid-90's and it's been in there ever since, for everything.
My 303 is quite slick from having been shot a lot, including LOTS of hunting loads. If yours is well broken in and not stiff, I'd suggest going to the 1Oz shot charge faster Sporting Clays ammo, I use the AA Super Sporting 1Oz. The extra speed and lighter recoil will help you more than than last 1/8oz of shot IMO.