have run into this a bunch of times
stacking the washers won't help as they flatten out to the same thickness once the bolt is tight regardless of how they're stacked. The design of the system is such that the thickness of the blade is critical. So is the dust cup. If the cup is missing the blades will NOT tighten properly, and will slip in operation, resulting in damaged blade, spindle, boss, eardrums, possibly emotional injury after the ol lady scolds you for leaving uncut grass in the yard
In 100% of the cases I've tried oregon blades on these mowers, 100% of the time the consumer was unhappy-and went back to Kubota blades
kubota makes a "double decker" mulching blade for the 60" ZD's, I don't remember teh part number but they work awesome. Much better than the standard mulching blades. The breakdowns and diagrams don't list them. I found them by asking someone at kubota directly, it is originally designed for a rear discharge 60" ZD3x deck.