Hah, I absolutely love how upset that car makes people. American car guys are like, "eww, Japanese motor!" and import guys are like, "ewe, Mustang!"
The RB26 is an amazing motor. The Mustang Fastback is an amazing car. I don't see anything wrong with putting the two together. If for no other reason that just to be DIFFERENT. Car customizers tend to get into habits and do the same poo over and over again, I'm all for mixing it up a bit.