I've been gradually putting together my '72 Dart with a 451 low-deck, AR Engineering motor plate, and Hedman B-body headers, and re-learned today how much fun that #6 plug is. As someone else posted above, a very awkward reach with a 13/16" wrench from underneath and fingertips on the plug itself is about the only way. All the others are relatively easy with the wrench, and I only needed a socket for #5.
I did study the inner fender for a while, considering breaking out the hole saw, but had some trouble deciding where to cut (how to line up accurately with the axis of the plug?). What diameter hole works the best? Just big enough to slide a 13/16" socket through, or a bit larger to allow the use of boot pliers...