The upper and lower rear coil spring retainer cups have weld nuts in them. The correct bolt size will thread into these weld nuts. Do you have the coil spring retainer cups? If not, a complete set, including the upper and lower cups and the necessary upper and lower hardware, is $20.00 + tax + shipping from many of the aftermarket parts suppliers, including:

www.lmctruck.com
www.classicparts.com
www.oldchevytrucks.com
www.truckandcarshop.com
www.bowtiebits.com