Overseas transport of cars and other vehicles is done on a large scale these days. Roll-on/roll-off ships are specifically designed for this type of transportation and they made their entry in the 1970s after shipping of cars significantly increased.
Uddevalla is only about 30 kilometers from Trollhättan. New Saabs were most likely transported by trains from Trollhättan to this small port city. RoRo ships did not exist in the late 1950s and new Saabs were hoisted onto ships by crane.
Uddevalla is still an existing port and they offer RoRo services, even though it appears to be at a small scale and mostly for used trucks and privately owned cars. In recent years when Saabs were shipped all over the world, the port of Gothenburg was used which is more suitable for the larger RoRo ships these days.
Related article: Port handling of Saabs at Port Newark near New York