My door is 15x27 so there will be some differences.
I used 2x3x3/16 sq tubing on the verticals where the cylinders are. (ends)
2x3x1/8 on the other verticals 81" apart
horizantal is 1 1/2x1 1/2x.07 42" apart
2x2x1/8 on the bottom truss w/2x3x3/16 angle braces
16" hinges w/1" pins on the ends.
9" hinges above each vertical in the middle positions
Building frame is 4x6x3/8 angle bolted to 4x6 oak timbers on the sides (about every 3')
Door frame is 3x3x3/8 angle across the top (angle on door lays against the angle on the building for weather seal.
Keep the weight near the bottom of the door light as possible cause that is the part that will be trying to pull the building over when the door is raised.
Be sure to tie the endwall of the building securly to the side walls (go back to the next building roof  truss at least)
You  will need at least 48" stroke cyl. 50 ish would be better.

On a 18x40 I think I would use 2x4 verticals............the other demisions should work.
Have fun,
John

(PICT2462 (Medium).JPG)

PICT2462 (Medium).JPG (58KB - 30 downloads)