Yeah that setup is pretty much like mine but without having to use a fuse box. I can see both lanes as well and don't use my mirrors anymore. I would say use the smaller screen so that it can rest on the wind shield so it doesn't shake too much. I have the 4.3" and I would have preferred slightly smaller just from an aesthetic point of view. Here's the link to the one I bought in SA:
http://www.bidorbuy.co.za/item/14460...Low_Price.html
It would be great if you could have a gps and rear view camera working off the same screen.