Blowby; I've been using those Fel-Pro gaskets lately with good luck even without additional sealer. As far as bolts go, I recommend the 3/8 headed or 12 point heads. Allens will be a nightmare if the wrenching area gets messed up - and they do. If the tube is too close to the bolt hole to allow for a wrench you can do the following - CAREFULLY. Clamp the header flange in a vice. use a ball ended punch of about 3/8-1/2" diameter with a hammer and carefully tap the tube wall in just enough for clearance. If you beat it in too much you could break the weld loose inside. Just do enough to accommodate the wrench. I worked in the Engineering Dept at Hedman Hedders for many years and I usually fit-checked the bolts/wrench clearance before I even started an installation. Nothing worse than getting the header and all the bolts in only to find you can't get a wrench on good enough to tighten it all up. NOTE; I've even done this to chrome headers with no ill affects, not sure about coated ones though.
When I bought the copper, I had it sheared into the width of the header so all I had to do was drill holes for the bolts and tubes.
The Jalopy Journal
