Well, there are a couple of threads running right now that discuss what type of aluminum the shells of AS in the 70's are made of. In '73, there is a good likelihood that your trailer is made of 6061 T6. The crux of the debate on the earlier mentioned threads is whether the Alcladding ended when AS switched over to 6061, or whther it is a 6061 Alclad--nobody seems to know for sure.
So when I redid my bellypan, I used 6061 T6 on a .032 thickness (non-alclad). It was tough (daunting) to make those curves where the pan wraps up to meet the shell. Luckily, I had bought an English Wheel on a whim, and that did the trick. I used 5055 on my interior, and that stuff is soft as butter--I don't think I would want to use it in an application where it might need ding-resistance. Whatever the completely curved banana wraps is made of, it is actually softer than butter. Mine were very dented, dinged, and deformed, but I got them straightened out. Tried to buff them a bit to see if they would take a shine, and concluded that there is no way (which, I think would argue against worrying about using Alclad on the belly skin).