Thanks Big Dave for the info... I have the assembly manual and it shows the location of one of the bolt holes for the tunnel cover
measured from the edge of the carpet. Since the mouse infested carpet has been removed, I can't get an accurate measurement.
Because the part of the shifter body where the handle attaches to it, needs to be basically centered in the 'hole' in the tunnel extension, locating that point from the interior of the car is risky.
One way to reduce the risk is to locate that point from underneath, using the actual transmission and shifter body, lifted into position, as a template. Mark, and drill a hole from underneath. From there cut a temporary hole in the tunnel (maybe 6"L x 4"w) for the shifter body to fit through.
Delay final cutting and installation of the tunnel until the transmission w/shifter body attached is in place. Fit the tunnel extension precisely where it belongs and mark the outline. Remove as much of the tunnel as you wish; making sure you leave enough steel to attach the tunnel screws.
I believe GM removed all the steel 'inside' the extension to avoid creating a pocket to trap water. Also, there is a water proofing 'seal' between the extension and the tunnel.
If you want to jazz it up a bit, you may make it look like my '63. That's a shifter plate from a '61 SS w/bench seats.