89 240sx ka24e mostly stock glowing manifold

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(14in electric fan short short ram intake intake port matching possibly a bad maf sensor plug but it does work universal 1 wire o2 sensor ticking lifters flywheel and other drivetrain mods) i was driving my favorite road the other day with a couple of friends and we stoped at the end and i poped my hood to make shure evrything was cool and i noticed that the (stock cast iron) exhaust manifold (with heat sheild removed as well as the emissions equuipment that sits next to it) was glowing both where the 2 sets of 2 came together (not the pipes just the iron surrounding the pipes) and also around the o2 housing. im guessing this is a sign of running somewhat lean (sign of a powerfull? motor) but kinda dangerous as far as blowing stuff up and melting critical parts not too woried but kinda curious. also if i can make the big thick cast iorn glow wouldnt i probably break aftermarket headers an what about the aluminum head? thanks for any feedback