They shouldn't be on the cap in order 6-1-2-3-4-5 or 4-5-6-1-2-3. 1-5-3-6-2-4 is correct. Start with 1, then take the wire from 5, then the 3... I once wired it backwards... 1-4-2-6-3-5 on a 270 gmc. The engine still ran, but not very well! It seemed as though only 3 cylinders were firing. I was still able to get my truck up to freeway speeds. it was running really rough and I thought it needed a rebuild, but it was my dailey driver and I was going to drive it till it stopped! 2 weeks later I realized my mistake and got the old girl running right.

GMC 6s are tough and forgiving motors!