I wouldn't add any spacers to the top, the center bolt is there to seat in the cushion, which seats in the perch, to prevent the axle from sliding on the spring.
Urethane cushions will help take up some of that room, as they don't squish out like chinese rubber.
Energy Suspension makes them, and there may be others. $40 on ebay.
1 example
https://www.ebay.com/itm/17543167824...0227fd64268358