The generally accepted minimum pellet count for a killing pattern is 100 pellets in a 10" circle at any given yardage.
If you are intent on sticking with lead I would suggest going to #6's and getting a choke somewhere in the .650-.660 range.
If you would like to switch to heavier than lead I would try Hevi-13 3" 2oz #6's with a choke in the .660-.665 range.
Of course you could just shoot Nitro #7's and be done with it! LOL
Good luck!