JC Whitnet used to sell, maybe still do, some bolt-on coil helper springs. Without seeing it, I can't say if its screwed up or not.
New springs are redily available. I checked
http://www.napaonline.com and found four listings for a '66 C20. I also looked here
http://www.classicparts.com/searchprods.asp?searchstring=springs&pagenumber=2&val=0 and found some more. I suspect most any aftermarket truck parts vendor will have what you need.
