With your HEI ignition you should run a wider gap than what the tune-up specs called for. The HEI produces a hotter spark which likes a wider gap for better ignition. Going from memory here but I think around .045" is about right. Set it and forget it. It's always the same regardless of how you intend to drive the truck. You always have to check the gap when installing plugs since the gap is easily changed by simply mishandling the box during shipping.