I have the same mirror quandary as you. I've decided to go with Velvac mirrors similar to what Keyair is doing. Our 310 mirrors which would be basically the same as what was on the Argosy motorhomes really suck. I feel they are to small and don't provide very good coverage.
I haven't opened up the side panel inside the Argosy yet to see what kind of support is in there for mirror mounting but from what I can tell it doesn't seem to be as substantial as what is found on the 345 and probably the 310 so that will have to be addressed.
To make things even more difficult the rub rail is closer to the window than on the 310/345 models. This means there is less room to mount a mirror base like a Velvac mirror uses. If you compare the two pictures you'll see what I mean.
Since I'm not home at the moment to get specific measurements I'm just guessing but I think in order to use a Velvac mirror I'm going to have to space the mount out about an 1" like Keyair is and then notch the bottom of the mount to sit on top of the rub rail. I don't think this will look to bad and from what I can see it seems to be the only good option.
I originally had planned on fabricating base extensions myself but I don't think I'm going to have the time so I might farm that work out to a guy down the street who has his own machine shop. If you would like when I talk to him I can talk to him about making 6 instead of 4 mounts.