72 Dart, mini tub, narrow rear, spring relocation

or go to an online metal shop and buy the through tubes. They will cut them to a length and you'll need to clean them up.

http://www.speedymetals.com/pc-3494-8242-1-od-x-065-wall-dom-steel-tube.aspx

That place will ship it via USPS as well if you call them.

You can buy larger OD tube as long as the ID is in the .875 range.