It's easier for me to do slappies on my setup with indy 139s than it is with my setup with ace 44s which are slightly wider and certainly looser, but I'm positive it's just because the 139s are way more broken in.
I really don't think the setup makes a difference. Learning where to put your feet (basically bolts) and how to quickly shift your weight are the key.
And yeah, if you're setting up a board JUST for slappies, you're probably just an old guy and are ok with the fact that your knees have had enough impact for the rest of your life.