I own both 804 Nautilus and 803D speakers. Both have exactly the same FST midrange (kevlar) driver, but different tweeters. I can tell you the Nautilus tweeter is dimensionally quite different than the diamond tweeter, and I don't think the Nautilus tweeter will mount properly on the 803D. They also have different sensitivities so you would have to alter the resistor in series with the tweeter, and maybe the coupling cap.
Sound wise, I liked the Nautilus tweeter quite a bit. I don't think they are as far apart as some would have you believe. But all that is mute if you cannot mount them on the top.
One thing you might look into is whether the separate driver parts are available instead of the whole tweeter. I think they might be, as the exploded view in the technical manual shows individual parts. No sense buying the outer casing if yours is fine.
Sound wise, I liked the Nautilus tweeter quite a bit. I don't think they are as far apart as some would have you believe. But all that is mute if you cannot mount them on the top.
One thing you might look into is whether the separate driver parts are available instead of the whole tweeter. I think they might be, as the exploded view in the technical manual shows individual parts. No sense buying the outer casing if yours is fine.