The first step is to hone the inside of the barrel to a mirror finish- a smooth blemish free bore doesen't damage the wad as much, so it doesn't open up as fast and will improve pattern density. The forcing cone is already somewhat elongated , but from the ones I have seen, it is also very rough and in need of smoothing and polishing.
This flies in the face of conventional thinking, but sometimes a sligthly more open choke will produce less fliers- especiall as you move up in shot diameter- too much constriction results in much of the shot being deformed, and deformed pellets spread out more quickly. The amount of shot also can contribute to this problem- you might find a 2 3/4" load patterns "better" than a 3 or 3 1/2" load does.

Duane....you nailed it and the only thing I would ad is to try a load with a little less velocity.