Hello,
I too looked at lead screws for 70", but the critical speed for a long unsupported shaft I calculated to be under 400 RPM which with 1/2" lead which makes it under 3ips. We felt it was not fast enough. Lots for force, but not enough speed.
This is a good reference for calculating force, and critical speed
http://pic-designcatalog.com/images/.../section_3.pdf