Gas and Oil Home Heating Furnaces - Remodelling - Duct Size Q's

I'm currently in the process of remodelling an older section of my home, and have some questions that I need help with regarding the duct work.

I have a hallway wall that contains all my heating/cooling ducts. The wall in a non structural and is framed with 2x6's to allow for the size of the heating ducts (4" round I think). I'm removing this entire wall from floor plan.

Problem - I can move all the duct work to another wall (I have a large crawl space) however the outside walls I want to move it too are only 2x4 walls. Can I use a different size of duct (maybe retangular) or something that will maintain the volume or air in the 2x4 walls.

Please help, this is almost the last issue I have to resolve.

codetoad:
It may be possible.

Read the before posting announcement and then come back to us.

is what you need. 3 1/4 wide by 10 will accomodate 6 inch diameter, 14 wide will accomodate 8 inch diameter. this will fit into the stud space you describe. outside walls are not the best place, as insulation is negligent once you fill the stud space with duct.