We were using 80% nitro and raising compression for more performance.  A new plug for a race or record attempt etc. so $8 a plug as well as performance mattered back then.  Now things are limited to 10% nitro so it isn't as big of a deal.  A sport engine is more geared towards a good idle and longevity.  Most plugs are pretty good now if the heat range is chosen properly.  Heat ranges vary by manufacturer as well, so it is experimentation.  Everyone is familiar with OS, so it is an easy choice to get the #3 or #8 because they are so popular.  They are OK, but a bit pricey.