Everything that Joe mentioned is OK - I made an error in calculation, my apologies
....
However, I do not think that you really need exactly 1 rpm - because barn door arm length can be calculated to suit the speed of the motor shaft (after reduction, as per images you supplied).
This can be done by moving the hinge into appropriate position - much better than fiddling with potentiometers, especially if one is not handy with electronics. Using the coarser threaded rod is another way to adapt slower motors to desired output)
The good supply of various gears and timing belt reductor stages are old printers (they are very easily found here in Oz, people are often throwing them on the grass in front of their houses).
I would simply go for EQ1 Single Axis D.C. Motor Drive (hopefully motor is also included in the kit?) and then see what I can do with available gears (from old printers) in terms of arm length.
Of course, check the website Joe suggested, for possible motor speeds. You can also up-gear your reduction (speed up instead of slowing down. This will increase the PE, but for wide angle photography (short focal lengths of lenses) it doesn't matter much)
EDIT: I will be back to you with detailed design parameters around EQ1 Single Axis D.C. Motor Drive shortly.