Ducting Systems and Air Ventilation - duct add on

I have gutted my home, and I want to run a return air vent upstairs. The only thing upstairs is one floor vent on the hallway floor. As you can imagine, the upstairs is hot in the summer. It’s not really practical to run a cold air vent in each room. I was told that I can run a large return vent up a wall, through an upstairs closet, and out into the hallway. I’m thinking about using two wall cavities up to the closet floor, and then use rectangle sheet metal duct to run it the rest of the way. I guess I could use wall cavities for the whole run, but it’s an outside wall, and that might be a lot of energy loss.How big should this return air duct be? 8"?

I also want to run three vents upstairs, and eliminate that floor vent {I hate that floor vent}. My problem is that my plenum has no trunk running off of it, and the side I need to add more runs to is full. My house is only 1200sqft. The furnace is an Aireflow 75,000. The upstairs run is 20ft,but will be eliminated, living room 4ft, dining room 8ft, downstairs bathroom 30ft, kitchen 24 ft. The downstairs return air is only 10 ft from the furnace.

Should I make the trunk just big enough for three more vents, or a little bigger? Should I run the upstairs off the plenum, and reroute the existing ducts from the trunk? I’m just concerned that I will have enough velocity to push that air upstairs, or if it even matters. This furnace and central air unit was installed in 2003 and the vents make a lot of rushing air noise.

I guess I could use wall cavities for the whole run, but it’s an outside wall, and that might be a lot of energy loss.
Sure would. Insulation belongs in outside walls, not ducts if there is any way to avoid it.

This furnace and central air unit was installed in 2003 and the vents make a lot of rushing air noise.
This tells me that the existing duct design is already inadequate.

Duct design is an art as well as science. There is not enough space here to tell you all the rules for duct design.

Thanks for the adivice on the outside wall.

I agree have the duct sized right and you will not have an issue.