Hello Michael - I have "0" experience shooting cattle with anything, but I like Alan's suggestion immensely.
Yogi has done excellent work on his double, as has Alan with his pelters.

If restricted to a shotgun, I would be very much inclined to go with load developed WW (wheel-weight - crimp on type only) cast round balls. I trust their penetration over any conical, however the Lyman sabot slug and those hardened Brenneke's sound as if they may have a future as well.

A bore size round ball, held in a 'cup' of some sort to help keep it centred in the bore is the way to go in my neck of the woods. Here, I only have grizzlies to worry about - no wild cattle - guess the grizzlies ate them all.
This 'cup' I speak of can be the base gas check portion of a plastic trap wad, or a similar item purchased from wad producing companies. Lyman shotshell handbooks give good loading data for slug loads. As the other's have suggested, nay, stated, do not attempt to shoot anything remotely dangerous with Foster slugs. Sometimes - sometimes, the 1 1/4oz Federal Fosters will work - sometimes. Winhester even as a high velocity 1 oz. to make it flatten into a pancake faster - oh boy! You want penetration. You already have size. So - a .715" RB at 545gr. pure, about 525gr. in alloyed lead (WW), to a .729" - about 580gr. travelling at 1,500fps will do the job - be it wild cattle, grizzly, Indian Elephant, tiger or whatever.

If you want the simplest of loading, use black powder - up to 7 drams in a 2 3/4" case. You can get this power level using black powder or it's substitutes, but trust me, smokeless powder kicks a LOT less giving the same power. With smokeless powder, you'll need to develope a wad column that fills the space between the powder and the base of the ball at proper crimp height. Lyman sells a roll-crimp tool that works in a drill press, or you can adjust the wad column to use a folded crimp.
I use a normal folded crimp with a slight 'twist'. I use a slightly high wad column so after crimping, the ball is seen in the middle of the crimp. That just makes it easy to see what the load is, shot or ball. Guess I could take a picture, eh?