Here's some photos of a weight bench I'm building for my son.  The frame is 2-1/2" and 2" square tubing.  The seat is from a Bowflex.  I made some bushings and drilled and tapped the seat mount tube to take 5/16" cap screws.  The welding was done with a Readywelder running off a Miller Dynasty 200DX with .023" solid wire.

Pete Keillor