Wow great work! It's nice to see a custom fab build that's kept so clean.. Definitely love the look of the wide rear end on the sedan! Literally looks stock..
As for motor swaps whether you go LSx or E9x, you're going to have to do something about the rear end.. The stock diff may not be able to handle that kind of power. Same goes for the halfshafts and any other component under heavy load/torque (Driveshaft, u-joint etc).. But otherwise, if you're capable of getting this far into the build, I don't see what's stopping you ! Would be REAL awesome to see a BMW swapped M3 V8 E9x motor in there.. Super original, and still true to its own. Would love to do this myself.. Damn..
Keep it up!