I was just wondering what I need to know about choosing the correct spark plug for my SBC 400- 40 over w/ alum heads and about 10 on compression. I found out that the Accel shorty header plugs that I was using were too short when I took the heads off. By this I mean the "reach" was not long enough to actually reach the combustion chamber, but it still worked. I actually need a 3/4" reach plug. I don't know how to figure the hot or cold requirements that I need. Any help would be appreciated.