Here's an ad from August '68 for Foxcraft traction bars. They appear to use the stock spring plate like the original Nickey/Bill Thomas bars did. There are some NOS Foxcraft bars for tri-five Chevys on ebay... http://www.ebay.com/itm/Foxcraft-NOS...item282e803162
I have a pair of those in the original box for Camaro-Nova. Bought them on eBay ten years ago for my '67 and never used them.