factory cast iron heads with tapered spark plug seats?
.055" is more gap than the suggested plugs are designed for, by a lot. the ground strap will not be parallel to the center electrode causing rapid wear and short plug life. the gap will not hold at .055" for long at all, but will increase as the electrodes wear.
RV15YC4 is designed to gap at .039" https://www.sparkplugs.com/product.aspx?zpid=8446
RV15YC6 is designed to gap at .059" https://www.sparkplugs.com/product.aspx?zpid=8576
but be careful these are extended projection tip plugs and may reach the piston?
how are you at reverse engineering? https://www.sparkplugs.com/Search.aspx?ptn=Spark+Plug