Sagging Spreaders

[VinceS]

I have a Corbin 39 with a LeFiell mast with double spreaders. The spreaders are hollow and have a sleeve end with a two point attachment point to the mast.
Clevis pins secure the spreader to the mast fitting. The problem is the mast attachment allows the spreader to move up and down rather freely. Attempts at seizing the shrouds to the spreader ends with monel wire are marginally effective. Typically by mid season one or both lower spreaders are sagging. I used a Loos Gauge to tune the rig to about 20% of the breaking strength of the wire. I have little to no slack in the leeward shrouds when hard on the wind.
What I thought I'd do this off-season is run a cable from the spreader end to the tang area just below the upper spreader. This would stop the spreader from sagging down but do nothing to stop the spreader from rising up. Then I thought about using a jaw & jaw turnbuckle to attach from the mast to the spreader either above or below the spreader. This would prevent the spreader from moving up or down and would be easily adjustable.
Do you see any problem with the turnbuckle idea? I'd still seize the shrouds to the spreader ends to prevent them from popping out of the notch.
All comments will be greatly appreciated.