5/16 wrenching header bolts

Redfish, ARP sells the 3/8" thread size with a 5/16" head size. If you look at the pics of the 3/8" head above you can see that it widens at the bottom of the head to increase the strength. I'm looking for 5/16" thread size with a 5/16" head size. Can't seem to find that one since it's not a chevy or ford bolt. Dust, thanks for the help, I will call and see if they will sell them or give up the source.