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This should help in identifying what you have and wiring it in. Yes you will more than likely have to at least pull the dash bezel out as most older items installed from the rear, can't confirm on yours. Most GM I've had it was a bezel, clear lens then a gauge pod which had the individual gauges you could remove. If you do yank it out it's good time to replace the lamps back there.
You must at least partially pull the dash. The stock tach does not have a power lead. One connection to the negative side of the coil, and a ground strap at the dash mounting, that is it. Here are the instructions for pulling the dash in the 70-72.