I was able to get 4.6 degrees of positive caster in my 70 Charger using poly strut rod bushings, STOCK upper control arms with the Moog K7103 offset bushings.
Another way to go, much cheaper and stock appearing:
Lower ball joint camber spacers/washers! AndyF makes them, Mancini sells them. They allow for more negative camber, which, along with the offset bushings, allow the car to get more caster without the camber going into the positive range.