It's a pretty common mistake to end up with 3/8" pushrods on a BBC engine. When looking around at most hi performance suppliers like Jegs or Summit, they carry almost nothing in 5/16" diameter, and tons of 3/8"-7/16" offerings. I didn't even see any 5/16" pushrods offered by Comp, so I'm guessing someone there (unkonledgeable) just spec'd the smallest they carried.
I'm running 7/16" Crower pushrods on my 454. I like the larger diameter, and my Edelbrock heads are setup for the 7/16" already.