The rule of thumb is the higher the compression ratio the smaller the gap. GM was all over the place with plug gaps going as high as .080". The HEI can and will jump that no problem but that gap was for pretty low compression engines. You would be safe around .045". If you are running the engine the way I described be sure the battery is well charged. The HEI needs a lot of voltage but if the battery can crank the engine that is enough for the HEI. Tune up specs