1969 Camaro Horseshoe Shifter install
 

I'm installing a horseshoe shifter and I am hoping someone can tell me the size of the hole I need to drill on the floor to run the cable through?

I measured mine with a micrometer. It was not perfectly round. It varied between 1.30" and 1.55" in diameter. The proper replacement cable has a grommet on it. You might measure the small outside diameter of the grommet to double check.