Agreed all need to be loose until all 8 are started. That was going to be first response as well. I suspect that's what you're doing, but just need to confirm. My GP Headers were that way. Each had to be dead loose to get all eight started. The bolts needed to go exactly straight into the hole or wouldn't start threading. Once all eight were started, then started snugging them down.

If that still doesn't work, I wouldn't be in a hurry to get out the tap. I would take it slow. I know it seems like going backwards, but I would remove enough bolts so the header can move a bit and you can confirm the offending holes really do have a problem. I suspect they don't. Likely you just need to open up the holes in the header a bit so all eight will go in.