Off the top o' my head, the intakes are different. The tip tanks are more obvious as the T-38 never had any. I'm not sure what else but I've read up on people converting them and they always mention the tip tanks and air intakes. The thing is, pretty much all of 'em do it! Airfix, Hasegawa, Starfix (heheh), and a few others too that I can't recall. They just box up F-5B's , do the box artwork as if it were a Talon, and do the same for the tranfers, and that's the end of it.