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Please forgive a novice question and not to hi-jack but Mancini's page is great but I see they list many plugs for the different comp ratios...

What is the difference between these plugs?
Does a "street car" not see much gain from this as I have always just used a standard auto parts store suggested replacement.
Or am I just not well informed?

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The basic difference is the heat range{length of the porcelin tip}...The over-the-counter plugs are fine for most street engines..It really comes down to a matter of preference...I've always used a champion plug {like the RJ12YCs,it's a "shorty" too} in a big block and have had good luck...Autolite,Accell,NKG all get good reviews and some swear by/hate them because of their own experiences... The general rule is start with a factory type plug for the application and see how the engine likes it...If it's burning clean and lasts thousands of miles there's no reason to change it...There's no magical plug that'll give huge gains over a clean running set-up....
Now,highly modified engines may need a plug with a different heat range depending on many factors including compression,exhaust,carb intake, etc...Plug reading is a real science and some are very good at it... In my experience a good number of guys seem to have other engine/tuning issues and want to blame it on the plug and start swapping heat ranges and/or brands labeling whatever plug brand they tried as "junk".. ....