Thanks everyone. I made a couple of 90-degree boots in place of the straight ones and that helped two of my three cylinders. #5 seems to want to touch regardless, and I can see where the previous owner "dimpled" the header to make room for a socket just to change the plug. I'll experiment with some heat wraps and "modify" the header as necessary I guess.

I love the BBC, but don't seem to recall some of these issues with my SBC...