Let me make a suggestion. If the collector is ceramic coated you don't want to drill or weld in that area.
As far as the entrance into the cabin, I came off of the floor on the passenger side about four inches forward of the seat and four inches above the floor. I popped right into the cabin thru the side carpet because I have a logger.
This area is very accessible in the tunnel from below by removing the belly pan. With a gauge you might want to come in a bit closer to the floor and keep the wires behind the tunnel carpet and bring them forward and up into the dash.
You can also come in anywhere in either toe box, but, be careful about running the cable parallel and in proximity to the collector since it runs about 600-700 degree's on a cool day. It is high temp insulation, but, not intended to run in the same cavity for any sustained distance.
If your dyno operator uses either forward oem O2 location for his data, put the permanent sensor on the opposite side. If you do not have cats the location is meaningless. Put it where it is easiest. Most exhaust system have a short transition pipe in the rear as the pipe makes its ninety to the inside. This can be pulled in five minutes on a lift, less hassle means more fun.