Time to replace the floorpans and brace.

The wiring harness in the car was pretty chopped up and cobbled together. It's been removed. I've got a Painless harness for a first generation Camaro, Painless says it should be a pretty close match. Wiring won't happen for quite a while longer though.
Dash has been removed.
This picture was really only for my later reference. I wanted to make sure I had documentation on where the seat mounts went.
Cutting out the old rear brace. I had read that this brace could not be removed without taking the body off the frame - but I don't feel comfortable taking this body off this frame without more reinforcement, so brace happens early.
New brace is in place.
This and the next picture are also intended for my own later reference as well.
More reference.
I almost forgot to weld in the seatbelt mounts before I welded in the brace. This would have been bad, the frame is right under the seatbelt mounts (welded nuts) and you can't get there with the brace on the body and the body on the frame.
I started cutting out the remaining floor, using the existing rust holes to save some time. I used a Sawzall, this was an incredible improvement over the cutoff wheel I'd used on prior projects. I did break one blade though.
Removed the column. I have not yet decided if I will reinstall this column or acquire a Flaming River or ididit column instead. If I reuse this column it will require a rebuild, it feels like the bearings are shot.
I had cut the top of the tunnel out where the brace went in order to properly install the brace. Then I welded the tunnel back together.
I set the floor pan in place and spraypainted around it in order to determine the boundaries of what should be cut out. I had intended to cut 1/2 inch inside the paint-line, but the rust-holes I used for faster cutting were not alway 1/2 inch in - in some places the distance up to 1 1/4. It will all still be lap-welded and have seamsealer applied.
Closeup of the rewelded tunnel.
Closeup of the front of the driver's floorpan. The pedal assembly has also been removed.
I had to unbolt the master cylinder to get the pedals out. It needs to be replaced by a more modern style (2-circuit) type anyway, probably power-assisted.
Closeup of the wiper motor mount. I had at one point thought that this car lacked the 2 speed wipers. I now know that it does, indeed, have them. (note: the close-up pic for this has been lost, so the pic is not clickable)
Back of the driver's floorpan has been cut out now.
Shifter hump removed, no further rust holes to use for cutting.
Test fitting. The red lines on the rocker are where the seat mount holes go.
I did go a little too high near where the accelerator pedal goes. Welded it back together.
Coated the exposed areas with POR-15
The remaining tasks are to drill the holes for seatbelts, seat-mounts, and plug-welds in the pan, and unbolt the body from the frame here at this bolt. The bolt is actually rusted to the cage nut, and the cage nut (and bolt) will need to be replaced. The only place I know of that sells new cage nuts is Ground Up Restorations, so I'll be getting some from them. Link directly to them: 1964+ Chevelle Body Cage Nut with Bracket - 6.95 each.
I suppose I'm at a stopping point on this floorpan until I replace that cage nut, since I'd have to pull the floor back up to put the new one in. Once this pan is done, the seat brackets get reinstalled, then the same process happens on the pass side. It should go a little easier over there, the rust was less extensive on that side.