I pulled the plugs out of the 455 today. Colour is good, just starting to turn a lightish brown and the gaps are all 0.033". Years ago I used to run rover V8's on LPG with the Hyfire 6 CDI box and often had the gaps out at 0.080" with no issues at all. The question I have is with the TA Performance HEI distributor should I widen the gaps.
I would go no more than .045. That's a good gap for the HEI, certainly plenty for 10.3:1 compression. Spark plugs come with a basic gap. You should not open or close that gap by more than .008" according to NGK. The reason being that the ground electrode will not be perpendicular to the center electrode if you open or close the gap too much. https://www.ngksparkplugs.ca/tech-info-spark-plug-faq.cfm#sp16 There are plugs that are made to accept bigger gaps. Our engines typically came with AC R45TS plugs. The AC R45TSX is the same plug with a longer ground electrode meant for larger gaps. Another example is the NGK UR-5. It comes gapped at .039. You could open it to .047, or close it to .031. If you wanted a bigger gap, the NGK UR-55 comes gapped at .059. You can open or close that gap by .008.
Excess gap is torture for the insulation of your plug wires, cap, rotor, and the varnish on the windings of the coil. You're unlikely to gain anything--power, economy, driveability--by widening the gap beyond .045, and even that's probably more than you need. Keep in mind that increasing voltage required means decreasing spark duration. The coil only has so much energy to provide. OTOH, it costs little to try it...unless you fail one of the above-mentioned parts. Make certain that you have a low-resistance button contacting the rotor, doubly-so if the coil is still in the cap. Last thing you need is more heat in the distributor/coil area.
