Can slab bacon be used as a sub for fatback

Yes, but the final product will be skillet cornbread with bacon.

I think yes, but the final product will be different. Slab bacon is going to be smoked and bacony, but you should be able to render out a cracklin-like, bacon-flavored product. If there is a lot of meat in the slab bacon (vs. a lot of fat like you'd get from salt pork) I'd only use half the amount called for.

Cast iron skillet cornbread with cracklins

What are you making?