Most likely the problem is a weak battery connection to the instrument panel area. The xtra current used by the HEI in your ignition circuit plus the current drawn by your directionals is causing a voltage drop in your battery feed harness. As a result your tach does not get enough voltage to work correctly.
Look for a bad/weak fusible link or a corroded fuse boxignition switch terminal.
Regards, George
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"...out to my ol'55, I pulled away slowly, feeling so holy, god knows i was feeling alive"....written by Tom Wait from the Eagles' Live From The Forum
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