Header Bolts for a 340

Locomotion said
3/4" seems to be the typical length with a 3/8" 6-pt hex head. Sometimes it can be more convenient to leave the studs in the end bolt holes because they go into the coolant. Just make sure the headers can move enough to slide off.

Or you can slip the headers in place, twist in a couple center bolts to hold them and the gasket in place and then seal and install the outside studs.