GM used to make and market very hard rubber spacers that fit under the front coil springs. If I remember correctly, they came in several different thicknesses, but have been discontinued. The part numbers are listed in the old Chevy High Performance Catalogs. I have two old catalogs at home from different years, and can check numbers tonight.
There is a set currently listed on ebay:
http://cgi.ebay.com/ebaymotors/64-72-GM-...1QQcmdZViewItem
Item 250013202756
I was thinking about bidding, as these things are a great way to fine tune ride height. I believe the overall height will be raised 2 times the thickness of the spacer.