There are three posts on the back of the Autometer Ultra-lite Tach I have, 1 is labeled SIG, one labeled +12v and one GRND. Am I correct in assuming the SIG goes to the TACH post on the distributor, the +12v to the ignition switched power source and the last is a ground?

"sig" should go to the distribitor,i believe thats right.

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http://www.fototime.com/inv/E695F2E9BD55636

http://hobbystage.net/camaro/tomstv

Yes, you are correct.

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'69 RS Camaro
355 5-speed 4.11