Theresa, you should call GM and ask for someone in the recall department as their was a massive recall on the trucks with dual tanks after the staged 20/20 segment. That should get you in the ballpark.
Go here:
http://www-tech.mit.edu/V113/N3/gm.03w.html
It sez "Of the 8 million trucks built between 73 and 1987 with this option, approximately 4.7 million were still on the road as of 1993. If you figure losing between 6-8% per year for what ever reason and you'd get close. Of course, these trucks are getting "Hot" again and folks are buying and restoring them.
Hope this is of some help.