LS3 should have a mounting boss and blade style MAF, in my experience. I've put in those crate engine ones from GM and they come with this style mounting boss.
https://www.summitracing.com/parts/nal-19166574
instructions are on page 2 in the bottom left for installing the boss:
http://www.chevrolet.com/content/dam...except-lc9.pdf
and the boss is directional.. that's important to note!