compression ratio is different on these engines and you need to change the lifters to get rid of the AFM. the L99 has a cam phaser and, to get the additional piston-to-valve clearance required when cam phasing changes, the pistons are different parts having valve reliefs machined into their tops. this would be good for a high lift cam.
10.7:1 (LS3)
10.4:1 (L99) because of the reliefs.