Are you sure about the crossmember being different? I just did an M41 for BW35 swap and had to put in, what I thought was, an M40 crossmember, but maybe it wasn't, because the BW35 crossmember was definitely different. You are aware that the crossmember attaches to the gearbox, and not to the OD unit, right? Your reverse light wiring goes through a relay, so make sure that is in good working order with clean contacts. If your reverse light switch is of the type that has the tiny coil spring inside an aluminum box attached to the rear of the gear box, you may find that fried, or not making contact for some other reason. Those switches are still available from your Volvo parts department. They are a pain to install, though. If you went from a BW35 to an M41, you may find my post on wiring late last month useful. When you are ready to hook up your OD switch and warning light, check out http://216.160.8.21/RWD/index.htm?id=695288.
Bob S.