Lyman's 3rd shotgun reloading manual has only 10 gauge magnum buck loads. However the The Mighty Ten Gauge Revision IV (1987) edition by Ballistic Products has several slug loads for the 3.5" 10 gauge shell.

These are described as heavy duty slug loads for bear, (in case one takes an interest while you are hunting waterfowl). One load uses a 12 gauge Brenneke slug inside a BPD10 wad. One is for a Slugmaster 12 gauge 437 grain slug. The last one uses "a special 755 grain slug" whatever that is.

You could also just try factory loads to see how they regulate, if you haven't already. You might get lucky. Some European gunmakers make side by side 12g shotguns especially for slugs. I think they might better regulated to shoot slugs, which have a different recoil, but also they may have different sights.