It sounds like your going to need to do some modeling to create the bridge and the tank.
Code wise it would be a little difficult because you would been to know how long the bridge would need to be to cross a river(
) or whatever.
I really don't know if it would be possible or necessary.
I would really talk to some modelers or other people who have created their own units first and try doing something a little bit easier like making your own car or something first. I really dont think a pontoon bridge would be necessary because there is very few places where you could use it.