I want to try and install a bathroom vent that no one installed in my house when it was built in the early 1900s, attached is the layout that includes a toilet sink and shower. Can someone help and tell me where the best location would be for me to install it. All feedback appreciated.

This is my first time at this so I'm not sure what I'm doing. I've done some reading here but I'm not sure which is better for sealing a pre-existing system. Some say the foil tape some praise mastic for sealing the joints. I just want to get it done. Also, what do I use to clean the surface?? The pipe is all sound, no rust. I will also be putting some roll insulation between the joist in the basement ceiling. Everything I've looked at shows the paper against the ceiling (i.e. upstairs floors). This leave the pink insulation showing on the basement side. How do I insulate the area where duct work goes through? I read where an insulted pipe helps heat, not only the floors, but the basement too. Can I staple plastic over it? My wood shop is down there and to leave the insulation exposed would be nuts. The problem is the ductwork extends way below the joist so I can't really put up any kind of ceiling. I don't want to do something that is going to cause a condensation problem either. I also have central air. Do I need to seal the return lines? I have a [B]new[/B] gas furnace and it draws the air from outside as well exhausting it in the same spot. With this kind of furnace, am I even using the return lines??