Effective modifications for the Abu Garcia 5600 reel to fish salmon include:
Replace the stock spool with a larger spool. Look for spools that can accommodate at least 250 yards of 20-pound test line.
Switch to a heavier fishing line that can handle salmon's weight and strength. Forty-pound test monofilament line is recommended.
Increase the drag system's power by adding more washers or upgrading to a carbon fiber drag system. This will help prevent the salmon from overpowering the reel and breaking the line.
Consider replacing the handle with a longer and stronger one that can provide additional leverage when reeling in a big salmon. A soft-grip handle will also be more comfortable to use during long fishing sessions.
Lastly, clean and lubricate the reel regularly to keep it in top condition and prevent damage caused by saltwater corrosion.