Not suprised you are having a problem with linear actuators that length.
I Have made my own with large Item Products extrusions, with Breco endless Timing belt transport, the extrusion allows the use of Chrome steel rod to be embedded on either side to allow a wheeled transport carriage or truck to be ran along the Rails, The timing belt runs inside the extrusion for one length and lays supported on the top to return.
I am not sure if this extrusion comes in the length you want, but you may have to construct some similar arrangement.
The belt has a precision gear reduction drive on one end with an encoder on the motor.
I have also used a commercial made linear actuator that goes to the length you want, The make escapes me, I would have to look it up, this was a project about 15yrs ago.
Ballscrew of that length would be expensive, you would have to contain the nut between a track of some sort to prevent whip.