Motor size depends on what kind of acceleration/deceleration you want. Ignoring your rolling resistance, which sounds fairly low, your 1 lbf pull will accelerate a mass of 1 slug (32.16 lbs) at 1 ft/s/s.
The equasion is Force = mass x acceleration so the maths is real easy.
You don't really need a lot of torque when your x and y are rolling rather than rubbing.
If you add screws you have to up rate your motor to allow for the inertia of the screw and it will be a fairly heavy screw over that length because you can't add a centre support.
Chains are a bit coggy and as soon as you add springs to compensate for wear you will get sag.
Long timing belts could be a bit elastic which is why most folk seem to opt for rack and pinion. The problem with rack and pinion is pi immediately comes in to the resolution equasion unless you get one of those fancy tooth pitches which cancel it out.