You are talking about the Firestone 500 Steel Radials which were all recalled due to catestrophic hgh speed tread separation failures. No one reproduces them, for good reason. I have the white letter version on my 73 Formula. Both the white letter and white stripe versions can be found via Corvette enthusiasts as they are commonly found as unused spare tires locked away in the underbody spare tire compartments of the 73-77 Corvettes. Caution: if you are able to get a set together, use them only for display. They are not safe for the street. Many people were killed in accodents caused by these tires when they were new. Imagine that same defective tire 40 years later and you are talking about juggling 4 hand grenades wih loose pins! Just google the phrase "firestone 500 recall" and you will get an eyefull.
Here's some links to ebay auctions. I bought my three NOS tires for $300 each. Some sellers are willing to keep the rims and take some $$$ off the price.
http://www.ebay.com/itm/Original-Corvett...b48&vxp=mtr
http://www.ebay.com/itm/Corvette-Firesto...895&vxp=mtr
http://www.ebay.com/itm/CORVETTE-1976-OR...bb5&vxp=mtr
http://www.ebay.com/itm/Firestone-500-St...2c7&vxp=mtr
An alternative is the reproduction Goodyear Polysteel Radial which was a tire that was used at the same time and is available via SmallHurst, one of the members here.