K&N is pretty much the only choice available. Part # RU-2780 for carbureted Triumphs.
K&N for 96 Triumph Speed Triple
They will work on just about any T300 Triumph including your T-bird.
Be prepared to sacrifice a little mid range power to these babies.
Just out of curiosity, your 97 T-bird should have had factory Kiehn carbs right? Is there some reason why are you switching to Mikuni's?