Hi,
Which type hump are you installing, 64/65 1st style consolette, or the 2nd style consolette/ no console or 66 Chevy hump which is what most use in the Grand Sports.
Also what trans are you installing as there are 2 different length transmissions, and at least 3 shifter locations.
T-10 early 27 spline output, note there is at least 2 tailhousing 1 has 1 pattern the other or GS tail has 2 mounting patterns, this trans is the same in overall length as a early Muncie's, M20/M21/M22
Later 71 and newer Super T-10, 32 spline output, is longer and similar in lenght to Muncies of the same time frame.
Your best bet is to install the trans with the shifter in netural, minus the handle and mark the underside of the tunnel, then center the tunnel panel around it, I used sheet metal screws to attach the hump, as later shifters have a bolt on handle ans if you weld it in place you can't remove without dropping the trans or at least that is the case with my car.
Either way start off with a small hole once you get the shifter and handle to go thru the gears mark the floor and do your final cut, it will look nicer than the factory holes that were cut.
Also do checkout that link, lots of good information.
Regards,