See the ponton spark plug article I preparted awhile back re spark plugs. I obtained the modern equivalents from Bosch Technical Assistance, so I am cautiously confident of their accuracy. I am planning a revision, but the changes are unlikely to affect your interest as expressed. Be very careful of conversion/crossing charts, including the one I provide. Errors long ago by others all too often get repeated and thus become--incorrectly--accepted as truth. I believe there may be errors in crossing the modern Bosch to some--but not most--NGK and/or Champion numbersin the chart I provided. The heat ranges do not directly/entirely overlap. The Glenn book has many spark plug errors and also confuses because Champion changed its numbering system in the mid-1960s. I have found modern Bosch regular plugs easily locally at my auto McParts stores. Cheap, so you can experiment to find the optimum one for your car.
http://www.mbzponton.org/valueadded/technical/sparkplug.htm
The Pertronix situation may alter your requirements. I have no information. What does Pertronix recommend re changing heat ranges?