Jack the car up real high, remove the top trunion bolt on the king pin and lower the A arm to point to the ground.
Stick the spring in, you may have to give it a nudge by hand or even a kick with your foot. Be carefull not to kick the car off the jack stands.
Jack tha A arm back up and put the trunion nut back on.
With the arm lowered, there is no pressure on the spring, this works just fine when you remove the springs too.
Spring pans never come off the A arms and it's a 5 minute job.