Avoid steel shot as it is too hard for safe use in early guns, and can damage the choke. Check all the major manufacturers - there are plenty of softer alternatives recommended for older guns. If the majority of your duck hunting is over decoys at 25 yards or so, you might want to test your patterns, and if too tight, a gunsmith can easily polish you choke open a little to give you Modified or Improved Cylinder patterns. It might hurt collectors value a little, but unless it is a rare example, there were a lot of these guns made, and would make the gun a lot more practical for your use...
Larry