I would consider rack and pinion. You can get 6 foot lengths of rack from Standard Steel Specialties, www.stdsteel.com/gear.htm for about $23/each in quantities of (2). Pinions can be had from Boston Gear, Martin Gear and Sprocket. Be sure to use 20 degree pressure angle gears. SSS has 20 dp rack as stock items.
With rack and pinion, there is no critical rotating speed, a limitation of leadscrews, especially when they are long.