Not sure about the extra rubber--shocks designed to work with one above and below washers--might get too much 'squeeze' with the extra piece. Also, believe there is a longer Monroe shock to use with the lower spring mounting, check that out. Cannot remember what I did years ago when raising the trailer as you have--did I change the shock or not? Danger is the shock not able to extend as far as the new mounting location allows when dropping, thus breaking the shock. Can't help on idea for extra rubber piece.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired