yes vortec heads require a longer threaded plug or the plug is firing in the threaded hole and
not in the chamber where it's supposed to be
check the vortec's what to know in the FAQ section for the correct plug from AC DELCO that may be your problem
http://67-72chevytrucks.com/vboard/s...ghlight=VORTEC
Special Spark Plugs For The Vortec Heads
R44LTSM, set of 8
MR43LTS, set of 8
P526S Accel U-Groove “SHORTY” Double Platinum Header Plugs approximately 3/16" shorter then R44LTSM plugs, set of 8