For interiors, you don't really have the choice, the 40mm only has a few mm shift i think, the 50 some more.
All the other lenses do great on the flexbody, the CFE 120mm is hard to beat in the studio, I also used the (very old ) c 60mm and CFE 80mm. The old black C 60mm is the only lens I was never really happy with on all my 'Blad bodies. At least get a CF or CFi/e.
Tom