The D750, D850 and D950 are dimensionally equal as mentioned, it should drop right in. The difference is that the 950 is a pressurized water system, whereas the 750 is thermo-syphon.
There is one other thing about the 950 that should be mentioned. It has a "true" alternator instead of the dynamo. I'm not quite sure, but I believe you may have to leave the left side panel off to make room for this.