It's not your header causing the problem, it's your heads. I have just run into the same problem after bolting a set of Performer RPMs on my Z with shorty headers. Before that I was running GM castings (several different ones) with the same headers and never had this problem. I ended up using Accel header plugs (avail. at most parts stores) and having to make a special tool (welded a third of a plug socket onto an open box wrench) to get some of them in and the others I had to use a short open box 5/8" wrench I also had to remove the plug terminal to get them to clear enough to screw them in on #2 and #7! The #2 and #7 cylinder plug wire connectors are a nighmare to get on and they touch the headers (just the rubber cover of the plug connector terminal with Accel 8.8 wires) but they have several passes at the drag strip and several hundered miles on them without arcing to the headers yet. If you find any shorter plugs let me know.