Hello,
first, it is important here to notice that i use Philip's Berlin branch which has some changes regarding placing of Bridges (as you can read here http://forum.simutrans.com/index.php?topic=13967.0). As a pakset, i use James' recent half-heights set.
When placing bridges (especially, but not limited to new possibilites), the different types of bridges sometimes work and sometimes won't. The Plate Girder Road seems to be quite versatile, whereas the Concrete Steel Cantilever Road has only a very few possibilites.
As an example:
At this post:
http://forum.simutrans.com/index.php?topic=13985.0
it is not able to replace one of the two bridges in the given savegame by a steel cantilever bridge, as he described it.
But other types of bridges are placeable.
Apologies for not having responded sooner: I have been somewhat preoccupied with being in the process of moving house. I am afraid, however, that I cannot assist a great deal, since I have not had a chance to test Philip's patch extensively and am not sure quite how it works.