The closed loop is the way to go. I have had TPI in my 71 since 87. I installed it with the s& p wire harness, but at the time not to many people were doing this. The hardest thing for me was the gas tank modifcation. You have to have a return line for fuel injection, and also didn't like the idea of a electric pump in the cab with me. Noisey too I would think, since you can hear about everything gm builds underneath cars and trucks. Make sure you get the heated oxygen senor if you buy new, that is a new break through since I did mine keeps the motor in closed loop all the time.
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Andy
71's Are for ever!!! or 67,68,69,70,72
http://geocities.com/chevy71tpi/main.html