I have a 3357, fun little gun if you can find good paint (they sell blowgun paint that's .50 cal, but quality is kind of iffy).
No one has ever managed to convert one to .68 cal as far as I can tell. The cylinder is no where near big enough. Plus the valve would have to be extensively modified to flow enough air to shoot the larger ball. The stock valve is very small.
Here's a picture of the 3357.
Not mine of course. Been meaning to take some good pictures of mine. It has a newer style grip since it was a conversion from a newer Crossman 357 pellet gun.