I don't know of anyone that has actually replaced the galvanized end caps on an Argosy before. With that being said, for the rear I'm not sure why you couldn't use the same 3 aluminum segments that were used on the Classic motorhomes. The shape of the Classic may have a little more slope to the curve where the Argosy seems to be slightly more rounded. However I don't think there are any ribs in that area so I don't think the slight shape difference would make a difference.
The front cap of the Classic is made of aluminum but I believe it is the same shape as the Argosy. What I don't know is if Airstream has the Classic end caps available or if the are now made of unobtainium.
Sorry I couldn't help any more than that.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)