The next step was to cut the hole in the floor for the manual shifter. I purchased a manual transmission console from Frank's Pontiac Parts (he's 5 minutes from my house!), and used it as a template to get a rough location for the hole.
I then proceeded to cut the hole out with a pair of tin snips. This was actually pretty easy, and gave a much better result than the factory hole, which was torch cut.
The new hole overlapped slightly with the hole that the automatic shifter cable previously passed through, so I made a small patch panel and welded it into place. I also welded up and ground flat the screw holes left over from removing the automatic shifter cable boot.
In the picture below I've overlaid the cardboard template I made for the patch so you can get an idea of where the overlap was:
This picture shows that the Tremec shifter location is very close to perfectly placed:
It's worth mentioning that I slid the hole slightly forward (maybe 1/2") from the "ideal" location I got by tracing the console. If there are any clearance problems it will be with the upper shifter hitting the front of the hole in the console, so I'm planning ahead in case I need to shift the console forward slightly. (I don't have the upper shifter yet so I can't tell for sure if it will hit the console).