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The temperature sending unit in the '72 heads was in the drivers side head. The temp. switch for the TCS (transmission controlled spark/temperature controlled spark) was in the passenger side head. This is a switch that closes when the temp. reaches 82 deg. F and enables vacuum advance.
As for the temp sender in the newer heads. I'd say it's not the correct one for your '72 gauge cluster, thus the incorrect indicated temp.
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'72 GMC SWB C1500 Custom, frame-off in progress. 383 SBC, 9:1CR, Comp Cams XE262H, Scat internal balanced crank, Eagle SIR 6" rods, Keith Black dish pistons, Dart Iron Eagle 72/180 heads, Weiand Stealth intake, Stewart stage I water pump, Holley 4bbl vac sec, TH350 with B&M Shift Improver Kit. 12 bolt positraction.   
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