I would frist try that hitch and see if you even need to add spacers. Some people have stated they did not need spacer and others stated they did. I was told directly by Hidden Hitch for the WK Grand Cherokee model hitches... that "washers may be needed to close the gap between the frame and the hitch."
If you do then just go to any hardware store or home depot and buy 12 (4 per bolt if needed, typically 2 per bolt will do the trick) grade 8 washers. ALOT of people with aftermarket hitches that have that gap use the washer trick to fill in that space.
def let us know how it works out for you!