Thumbs: You will find data for the Lyman 525 sabot slug in the Lyman Shotgun reloading manual. There is data available online at the Hodgdon website for the Lee slug and for slugs of the same weight as the Lyman, 1 1/8oz.
Cylinder bore on a 12 ga. rifled barrel is nominally 0.730". If you are going to shoot a bullet that is nearly a quarter of an inch smaller, you are going to need a sabot designed for the job.I believe you can load a .500" caliber bullet in a wad to shoot out of a rifled barrel. This is an area I'm just learning about as well.