340 head torque requirements

15 ft/lb on the rocker shaft bolts. Can be grade 5 or 8.
That is because they are not being torqued to the max available for that diam bolt. ARP lists 5/16-18 torque varying from 26 to 32 depending on bolt grade.
Tq is low because you are tightening on a hollow tube that can get squeezed out of shape with excessive tq. Not a problem with stamped rockers, but could bind up other types.