my only suggestion is don't do it....upsets the flow of the styling of the bike (sorry guys). 21" wires only work on certain particular designs and the does not include all choppers.
Plus depending on the height of the tire used it can mess with your handling......
Much like the big ass wide tire on the bike theme looking so ridiculous (hear me OCC?) the solid bagger giant ass end with a skinny 21 up front is silly.
All of course IMHO