Non-Adjustable Rear Shock Adjustment

I would like to raise the rear about 1/2 to 3/4 inch. The shocks are the stock (Unique) non-adjustable types. I'm hoping that a rubber spacer of some kind placed between the spring and the lower perch will do the trick. Any thoughts? Input from the Weavers would be nice. Thanks. By the way, won another trophey last weekend at one of the larger car shows in Carson City. The Weavers sure make one fine Cobra.

I spacer between the 2 top shock mount brackets will raise the rear, a 1/4" shim will raise it about 1/2".