Pretty sure the #6 plug is blocked by the exhaust not #8. Can you tell if it's the same as the other plugs? Going with self limiting plugs means you need to absolutely know what all 8 plugs are before you increase glow time.
Here's #8 without anything in the way. You can visualize where the Down Pipe and the exhaust manifold will give the challenge. My first go around (with SSD Insta-(Not so much) Heats) I got to it from underneath under the cab with a lot of "Army Speak" type words.
For Reference, Heat shield tube locations for 6 & 4 (right to left) on the exhaust manifold. Number 8's position can be approximated to the left.
Here's the exhaust all assembled (Fender and Inner Fender removed). You can see how the down pipe is in front of #6.