I keep mine next to the passenger seat, it might fit on the driver's side but have not tried it.
Edit: Just checked, it will fit on driver’s side (bucket seats).
After market O2 sensors seem to be garbage. If you probe between A&B of the connector on the sensor it should read 5-7 ohms, aftermarket (NTK, Bosch, Amix-Ada) ones are reading only 4ohms from what I’ve seen. Find an actual NOS Mopar one.
Correct, you have an open system since the radiator has a pressure cap and the bottle should just be an overflow bottle, should not be pressurized. Closed system radiators do not have a fill neck.
Edit: maybe technically it is closed if both caps are pressure caps.