Pickup tranny noise

The column-shift manual 3 speed Saginaw tranny in my half-ton '80 chevy pickup has been getting steadily noisier for the last few months, and has done its work well. Should I pull it and replace bearings, or just find a junker and swap it out? Also, is it possible or desirable to switch to a four on the floor, if it will gain me some better fuel mileage?