Don't buy the mancini stud and hold down kit. I did and while the hold downs were great the studs were horrible, most of them were not straight and the slot for a flat head screwdriver in the end of them was soft and tore up easily. Hold downs were awesome though.

Sorry I am no help on the 5/16" studs. I have bought ARP 3/8 studs for some race W2 stuff but I suffered through the mancini wobble studs for my old LA heads.