All joking aside, almost anything can be done if you have a shop that is incredibly gifted at fab work and a somewhat unlimited budget. Also, there is a MASSIVE difference between running a RWD engine in a transverse mounted position vs. running it in a longitudinal mounted position in a FWD chassis! Considerations would have to be made for (RWD vs FWD),: custom-tubed front ends, firewall modifications, modified fender walls, custom bellhousing, custom length axles, custom driveshaft, customized underbody, engine and transmision mounts, custom rear suspension, possibly custom driveshafts. It would get rediculously expensive and in a hurry! But, apparently it has been done before....with an SR anyway.
Link (Edit: Apparently @White steve fro CT beat me to this link, but I missed it as his text was posted with invisible ink =] )
It might even be easier to do a mid/rear-engine Sentra with an SR engine. I suppose you would have to modify the front suspension to work in the rear of the car and then do a whole bunch of fabrication on top of that. Our very own Dave Dunn
@OnTheChip has done this in a 1994 Acura Integra w/ mid-engine H22 swap. If you have not seen the thread, it is epic. I warn you though, It will take weeks or months to read in it's entirety.
Dave Dunn build
Link
It's not that it can't be done, it's just that it's so damn expensive and custom that you will not find off the shelf parts for it, which means that [S]almost[/S] nobody has done it, which means that it is hard to have a discussion about it, as the custom parts have yet to be designed. It would be a labour of love, fraught with an enormous amount of trial and error. Parts would have to be designed on the fly and then modified to work with the build specs as they would inevitably change during the project.
If you search the car forums for the various manufacturers, I
guarantee that you will find a nut or two in every corner. The nuts are just harder to find in the smaller FWD/SR Nissan community.