Having located the driven end cog correctly move the free end frame so the cog is central with the frame horizontal and slide the frame onto the cog. Then fit the frame onto the bed of the car but don't glue it yet. Holding the frame down with a finger make sure the pot moves correctly and reasonably smoothly. If not you can remove it and adjust/file/trim as needed. When happy with the action glue the frame in place and continue with final assembly and details.