You could use the 6x2, but I would personally go with the 4x4, thats what I have, and I welded plates on the topside of the tube, big enought to weld 5 inch pipe onto it, for the stacks[img=500x375]http://www.pavementsucks.com/forums/attachments/200507/attachment-837111.jpg[/img][img=500x375]http://www.pavementsucks.com/forums/attachments/200507/attachment-837112.jpg[/img]