Leaf sells a mask for the 645 AFD that shows your sensor area.
Yeah, I like the Mamiya AFDII. The problem now is finding a back I can afford that works with it and is physically robust. Aptus has a nice look to the files, but Phase seems tougher and better integrated (and no fan).
My experience is that the physical properties of electronic hardware count a lot: Mac Powerbooks for instance stay usable in their original configuration for years and become symbiotic with their users who cling to them even when outdated. My 1Ds proved tough as hell and I ended up taking it everywhere.