Header Bolts for a 340

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.
Thanks for the response. So 5/16-18 x 3/4 and I plan to use those nifty Nord-Loc washers.