What I do is have the axle in the car with the lower arm bolted to inner trunnions. I put the spring in place with a trolley jack . I lift up againt the weight of the car. With the engine in place it will compress the spring enough to allow the king pin to connect the top arm together .

Without the engine, I use a spring compressor on each side of the lower arm hooked under the lip on the arm and onto the axle with the top hook.
I screw them both at the same time evenly,together and the arm moves up enough to connect the king pin. With the jack underneath it's perfectly safe and quick and preferable to the Mercedes method of using a couple of long bolts and guides to pull the lower arm (connected to the king pin ) to the trunnion bolt mount .
Pontons are dead easy,I just use a steel bolt 18" long x 5/8" thread. The top end is turned down to fit through a hole in the top of the axle where i screw on a nut with a washer.
The bottom end is threaded for about 6Inches. I put a plate (a toyota land cruiser leaf spring actually, 100 mm x 50 mm x 10 mm ... ) on the bolt and screw a nut so the arm and spring are compressed up enough to connect the king pin.
You can use mild steel althread although Hi tensile rods are available It's not necessary,just use light oil to lube the threads and you will be OK.