I say .040~.045" and in a nutshell here's why: your HEI ignition is largely about higher spark voltage and more energy at the plugs or maybe I should say that was indubitably one of the main tenets of the whole design. The fuel we get today isn't what we got in 1978 and that should obviously be important to think about also. Some parts of the USA had unleaded fuel by then but not ethanol fuel. So, for one thing, ethanol blended gas runs leaner and this is another difference not contemplated in 1978. And ethanol laced gas will have water in it. We've discussed this point at length here already. One of the nice things about a slightly wider spark plug gap is this...whatever is in your carburetor has a much better chance of getting burned and especially if your fuel is leaner (which ethanol and water will conveniently accomplish for you). Another way of looking at this is a wider plug gap will add to or promote a more healthy ignition of whatever you are burning. And please trust me---the fire coming out of your new HEI distributor and HEI wires is plenty capable of hopping across a .045" gap. .035" isn't much different than what you'd be using with points, is it? Let's think a bit more about that---.035" is about the thickness of 2 sheets of copy paper wider than you'd have with a points distributor type plug gap of .030". So with a .035" gap, I'd say you haven't improved your situation much at all. I have a 235 and a Langdon's HEI and I use a .045" gap. Starts easier, runs better and burns more of the fuel (increased power). Isn't this what you wanted with an HEI in the first place? Good luck...
