At some point GM went to a larger U-bolt (from 7/16" or 1/2" to 9/16") You just have drill out the holes to fit the new bolt. Probably why the part# isn't correct for our year.
I just put the stock u-bolts back on my '67, they definitely
look smaller than those in the picture.
LMC has them listed for 12.95 ea.
I'm definitely going to get some of the 9/16" u-bolts. The stock ones weren't in the greatest shape anyways and no one needs a lower a-arm flying off.