The angle iron should keep it from cupping, but not sure how it would stop twist or bow. I added a base cabinet and top to the left of my radial arm saw to support long boards a couple years ago. My saw is on legs and about 5 inches above normal counter height, so I had to raise the level of the supports above the top of the cabinet. I made two 3x3 beams which are hinged at the wall and supported above the counter top near the outside end with carriage bolts screwed into T nuts. They're adjustable on both ends and can be pivoted up and out of the way when not in use (though I seldom do this). This turned out to be beneficial for two reasons - It's easier to adjust a couple small pieces to be level with the saw table, and I don't have to remove all the crap that accumulates on any horizontal surface in my shop in order to use the saw.
These issues will be even more important with the size slab you're going to use. So I amend my recommendation. Set the slab at least 3 inches below the level of the saw table and use temporary or permanent supports to raise long boards. This will allow easy adjustment should the slab move after installation as well as better use of the surface.