Mopar Muscle did a write up on building up a 340 with a set of "J" heads that were extensively ported. They found that the Denso Iridium #IQ24 actually netted a gain of up to 8 lb-ft. of torque through three dyno pulls. Here's the link if your interested:
http://www.moparmusclemagazine.com/techarticles/engine/mopp_0211_340_x_head_buildup/viewall.html