My old neighbor who had a pair of 53xs, one of which he engine swapped with an inline 6 from an M3 explained, to me once the subtle differences between the inline 6 engines of that same time era. There are just enough to make engine swaps very arduous between them, but it is technically the same engine. The upper intakes vary, for example. It is kind of a crap shoot.
This M3 engine he dropped in looked like it was supposed to be there, but he had to do a ton of supplementary things like the radiator, the fans, and a lot of electronics to get it to work properly. But it did work.
The answer is they are not just straight bolton, but you could make them work if you had the desire and knowhow, and were not afraid of being out of warranty. Most people get stopped by one or the other.
Personally I think a turbo setup would be pretty easy to do on the I6. There is plenty of room in that engine bay, and the stock manifolds merge reasonably close to the heads.
