Wire gauge size for power windows?

There are too many unknowns to give you a good answer. DC motors tend to pull the current to drive the load. More load, more current. If the supply voltage is low due to voltage drop, the current increases, resulting in more voltage drop... voltage drop increases both with wire gauge number and length. At some point the motor will stall, and brushes and windings burn. Modern motors have protection circuits, to limit current.

The Ram was designed for power windows, so the track friction, window weight and gearing were likely designed for that. If you are using to rig up for an a-body, the electrical load may be much higher.

One way to get an estimate of the current is to measure it, then the wire can be sized for length of run. Or use the #12 and see what happens.