Adjustable spring spacers are available at speedwaymotors.com. There is 0-2.5" and 3 1/8"-5 1/4" models.

Small(0"-2 1/2")P/N: 91645551

http://www.speedwaymotors.com/p/7038...-Standard.html

Large(3 1/8"-5 1/4")P/N: 91645552

http://www.speedwaymotors.com/p/3389...-Extended.html

You should get an extra pair of rubber or urethane coil spring isolators
to fit between these and your existing coils to prevent any metal on metal wear.