Ball or bullet will somewhat depend upon the rifling twist.
I have three 12 bore rifles and one has a 1:96 (slow) twist while the other two have a 1:52 twist. The slow twist one for sure likes round ball loads. The faster twist rifles do fine with a conical from Accurate Molds or a round ball.

Personally I’d get either ball or bullet sized at .750 and be happy. With soft(ish) bullets they will bump up to fit the bore and you have nice deep rifling to grip the projectile as well
Do your ball or bullet from 1-40 to 1-20 tin/lead, they cast much nicer than pure lead.

You will need to figure out if your rifle is a brass case or paper case chamber. With your groove diameter I would guess brass case. You would know which right away if a ball in a paper case wont chamber (thicker case mouth).
If you go with brass cases I’d suggest avoiding the cases from Track of the Wolf. The turned brass have a sharp 90° junction at the web and fail very quickly at bore rifle pressures due to the stress point.

Have fun,
Mike