I have the 123 on my 68 280 coupe. Everything stays the same in regards to timinig. Use the factory specs. I would put the engine at TDC. It will make it easier to set up the timing on the 123. Use a remote starter to rotate the engine. Overwise follow the directions.
I also removed the resistors.
Good Luck

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Britton McIntyre

68 280 SE coupe 'Hairball'
70 280 SL
71 280 SEL - RIP May 2010