The crankshaft gear lines up twice, once with the #1 cylinder in firing position, and the other time when #6 is ready to fire. Install the new gear, and align the timing marks. It will run fine! Just make sure the distributor is timed to fire the #1 plug when the valves are closed! You might have to re-time the distributor to match the new cam position, but it's not likely. The engine (pistons, etc.) doesn't care whether the piston is on TDC on either stroke, it's the cam and distributor that determines which cylinder is ready to fire.
Jerry

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