Hi all,
thanks for your replies and links. I already started doiing some stretching and core strengthening exercises. Last week I cycled next to a building that had lots of windows so I saw myself biking and was able to see my posture / fit when biking. The angle between arms (upper part) and body was less than 90 degrees while my hands were on the brakehoods. I was able to get a 90 degree angle by curling my back - by bending it.
I'm considering to buy a Nitto Technomic stem as this will fit my classic bike. If I get a 120mm stem the handlebars will move about 40mm to the front and 10mm down.