Good responses in this post, could add, unless you are running an electronic ignition system, you should use a conventional plug as previously stated, I recommend the Bosch Super-Copper Core, over here they are cheap. One test: get the car all warmed up on a drive and then disconnect the breather hose coming from the valve cover over to/at the oil seperator with the car running and rev it up, see what comes out of the hose, you're looking for excessive blow-by. Have you cleaned the oil seperator? It designed to funnel blow-by oil back to the crankcase but after thirty plus year it could be full of gunk and malfunctioning, remember at the bottom of the oil seperator is a tube that vents to the airbox, check out the inside of the airbox. Also I'd check to make sure all the exhaust manifold nut a totally snug tight, say they were not tight on the 2-3 branch exhaust manifold, it would diminsh the breathing for the combustion process. I'd say the brown color on your plugs is correct and indicates a good mixture. Too lean can kill a motor fast, too rich will kill it slowly. Wes says the injectors rarely fail, but who knows?