Well the point of the x pipe is to improve scavenge and balance the exhaust flow side to side. Installing them before won't affect anything when they are closed but open it, it'll defeat the purpose of the x pipe and the exhaust, like any moving air or liquid will find the path of least resistance.
Yes, I installed a set on my 78 powerwagon, after the X pipe. Been there since 2003/2004 ish.
Can they go in front, probably, I'm no expert by any means, that's just what made sense to me when I thought about it
