what I did with the rear of my 18 ( it was too soft and seemed too low) was to use some 21 shock linkage plates,I turned the plates around so that the distance between were the lower part of the shock bolts on and were the tie rod bolts on was at its greatest and it has made a big difference to the ride height and how stiff the shock is,I cant get over how well it now handles.
There is a template on this site for a 18 F3 shock linkage plates,I know thats no help but if you look at template you could draw you own,Just increase the distance between the lower bolt hole and the other 2 then get some made up.