"Reach" is the length of the threads, and does not include the distance by which the electrodes project beyond the threads:
It is crucial to use only spark plugs having the correct reach (threaded length). Using a longer-reach plug to push the electrodes further into the combustion chamber is right out of the question, for the threads' sharp edges will incandesce and cause a lot of preignition (and you might well smack a piston). Furthermore, the threads will collect carbon and make the plug difficult or effectively impossible to remove!
On the other hand, using a plug with too short a reach is also "no", because the electrodes will be recessed into the plug hole in the head, shrouding them and working against you, and carbon will build up in the head's plug threads, creating its own problems. Other notes:
Suffix numbers on NGK plugs (such as -11 and -13 and -15) indicate wide, extra-wide and extra-extra-wide gaps, and no, you can't just close down the gap on a wide-gap plug to wind up at a standard gap.
www.ngk.com is not NGK's site, it is the site of a vendor of NGK spark plugs.
The '75-up slant-6 head requires plugs with 14mm thread, taper seat, and 0.460" (11.7mm) reach. No longer or shorter reach is acceptable!
LZTR5AGP has a 0.984" (25mm) reach. No go.
PZTR5A-15 has a 0.708" (18mm) reach and a 0.060" gap. No go.
LZTR4AIX-11 has a 0.984" (25mm) reach. No go.
PLZTR4A-13 has a 0.984" (25mm) reach and a 0.050" gap. No go.
There does not appear to be a taper-seat plug of the correct reach with the extra-long electrodes.
