5.9 magnum header bolts

Burlapen said
I would be afraid of clearance from the bolt head to the pipe more than anything. Most 3/8 bolts will probably have at least a 7/16 head. It gets tight on a couple of tubes.

Yea, like where some of the tubes cross back over the top of the bolts.
They are already tight with 5/16 (which I have never had a problem with 5/16)

A person could get away with it with 3/8 allen bolts, but cutting the allen wrench down to get in under the pipes is going to be mandatory.