There are many old threads about mounting headers. In this post I have a picture of the original hardware that you should still have.
http://67-72chevytrucks.com/vboard/s...ds#post5714976
Including the pins 68Sunshine mentioned that should be removed, there are 8 threaded mounting holes. Two of the holes originally had threaded studs. I put threaded studs in all 8 holes. It makes assembling all that stuff to the side of the engine easier and provides more consistant clamping.
I robbed extra studs and short clamping bars from junkyard engines. The short clamping bars are easy to grind down on one side to allow for different flange thicknesses. Don't grind any material from the flanges.
ARMY Avionics here. Was stationed NAS Corpus Christi for 5 months before Nam. Got to see how you NAS guys lived. They ate on plates! Same with the guys at Clark Air Base, Philipines, but was only there a couple days.
Hedman, early Clifford without water passage built in. They're on the outside so it shouldn't matter.